diff --git a/backend/src/app.ts b/backend/src/app.ts
index 46957b5..9f06e67 100755
--- a/backend/src/app.ts
+++ b/backend/src/app.ts
@@ -19,6 +19,7 @@ import filtersRouter from './routes/filters';
 import customersRouter from './routes/customers';
 import passkeysRouter from './routes/passkeys';
 import passkeyLoginRouter from './routes/passkeyLogin';
+import googleAuthRouter from './routes/googleAuth';
 import publicRouter from './routes/public';
 import cartRouter from './routes/cart';
 import shippingAddressesRouter from './routes/shippingAddresses';
@@ -105,6 +106,10 @@ app.use('/api/customers/me/passkeys', passkeysRouter);
 // Unauthenticated, unlike the router above: this is how a customer becomes
 // signed in, so it cannot sit behind requireCustomer (#39).
 app.use('/api/customers/passkeys', passkeyLoginRouter);
+// Its own prefix rather than under /api/customers: this is the one route a
+// third party redirects a browser into, and the callback path is registered
+// verbatim in Google's console (#341).
+app.use('/api/auth/google', googleAuthRouter);
 app.use('/api/customers', customersRouter);
 app.use('/api/client-errors', clientErrorsRouter);
 app.use('/', publicRouter);
diff --git a/backend/src/google/oauth.ts b/backend/src/google/oauth.ts
new file mode 100644
index 0000000..6f72b23
--- /dev/null
+++ b/backend/src/google/oauth.ts
@@ -0,0 +1,254 @@
+import crypto from 'node:crypto';
+import type { GoogleConfig } from './config';
+
+/**
+ * The OpenID Connect authorization code flow, as far as Google implements it (#341).
+ *
+ * ## Why the code flow, and not the one with a token in the browser
+ *
+ * The browser is sent to Google, comes back carrying a code, and *this server*
+ * exchanges that code for tokens over its own TLS connection. The customer's
+ * browser never holds a token, so nothing that can read the page can steal one.
+ *
+ * PKCE goes in as well, even though this is a confidential client that holds a
+ * secret. It costs one hash and it closes code interception outright rather
+ * than resting the whole flow on the secret staying secret.
+ *
+ * ## Why there is no JWKS fetch here
+ *
+ * The id token arrives on a direct TLS connection to Google's token endpoint,
+ * in the response to a request this server made. OpenID Connect Core §3.1.3.7
+ * says signature verification MAY be skipped in exactly that case, because TLS
+ * has already established who answered and that nothing altered the reply.
+ *
+ * That removes a key fetch, a cache and a rotation path from the auth code,
+ * which is a real saving in the place least worth having moving parts. It
+ * removes none of the claim checks: those are what stop a token minted for
+ * another application, or for another attempt, being accepted here. See
+ * `verifiedIdentity`, where every one of them is enforced and none is optional.
+ *
+ * The moment an id token reaches this code from anywhere other than that
+ * response — a redirect fragment, a request body, a header — this reasoning
+ * stops holding and signature verification becomes mandatory. Nothing does that
+ * today, and nothing should.
+ */
+
+const AUTH_ENDPOINT = 'https://accounts.google.com/o/oauth2/v2/auth';
+const TOKEN_ENDPOINT = 'https://oauth2.googleapis.com/token';
+
+/**
+ * The only scopes this asks for, and the reason publishing needs no review.
+ *
+ * `openid` produces the id token, `email` carries the address and the
+ * `email_verified` flag the linking policy turns on, and `profile` carries the
+ * names used when an account is created. All three are non-sensitive; adding a
+ * sensitive one turns publishing into a verification review with a video
+ * walkthrough and a wait measured in weeks.
+ */
+const SCOPES = 'openid email profile';
+
+/**
+ * Both spellings Google issues for the issuer claim.
+ *
+ * It really does use both, and accepting only one produces sign-ins that fail
+ * for some customers and not others — which is about the least diagnosable
+ * failure this flow can have.
+ */
+const ISSUERS = new Set(['https://accounts.google.com', 'accounts.google.com']);
+
+/** A little slack for clock skew between this host and Google. */
+const CLOCK_SKEW_SECONDS = 60;
+
+/** Who Google says signed in. Everything here has been checked. */
+export interface GoogleIdentity {
+  /** The subject claim: opaque, stable, and the only safe identifier. */
+  sub: string;
+  email: string;
+  /** Whether Google asserts the address. The linking policy turns on this. */
+  emailVerified: boolean;
+  firstName: string | null;
+  lastName: string | null;
+}
+
+/** The claims this cares about. Google sends more; none of it is wanted. */
+interface IdTokenClaims {
+  iss?: unknown;
+  aud?: unknown;
+  exp?: unknown;
+  sub?: unknown;
+  nonce?: unknown;
+  email?: unknown;
+  email_verified?: unknown;
+  given_name?: unknown;
+  family_name?: unknown;
+}
+
+/** One attempt's secrets, minted at the start and spent at the callback. */
+export interface AttemptSecrets {
+  state: string;
+  nonce: string;
+  codeVerifier: string;
+}
+
+function randomToken(): string {
+  return crypto.randomBytes(32).toString('base64url');
+}
+
+/**
+ * Fresh secrets for one sign-in attempt.
+ *
+ * `state` proves the callback belongs to the request this browser started.
+ * `nonce` is echoed inside the id token and proves the token was minted for
+ * this attempt rather than replayed from another. `codeVerifier` is PKCE.
+ *
+ * Three separate values rather than one reused three times: they are checked by
+ * different parties at different moments, and a single value would mean
+ * anything that learned it from one check could satisfy the others.
+ */
+export function newAttempt(): AttemptSecrets {
+  return { state: randomToken(), nonce: randomToken(), codeVerifier: randomToken() };
+}
+
+/** The S256 challenge for a verifier. Google supports S256; plain is not offered. */
+export function codeChallenge(verifier: string): string {
+  return crypto.createHash('sha256').update(verifier).digest('base64url');
+}
+
+/** Where to send the browser to begin. */
+export function authorizationUrl(config: GoogleConfig, attempt: AttemptSecrets): string {
+  const url = new URL(AUTH_ENDPOINT);
+  url.searchParams.set('client_id', config.clientId);
+  url.searchParams.set('redirect_uri', config.redirectUri);
+  url.searchParams.set('response_type', 'code');
+  url.searchParams.set('scope', SCOPES);
+  url.searchParams.set('state', attempt.state);
+  url.searchParams.set('nonce', attempt.nonce);
+  url.searchParams.set('code_challenge', codeChallenge(attempt.codeVerifier));
+  url.searchParams.set('code_challenge_method', 'S256');
+  // No `access_type=offline` and no `prompt=consent`, deliberately. Those ask
+  // for a refresh token, and Google is being used to answer one question once —
+  // a stored refresh token would be a long-lived credential with nothing to
+  // spend it on and everything to lose if it leaked.
+  return url.toString();
+}
+
+/**
+ * Trades the code for an id token.
+ *
+ * Returns the raw token rather than parsed claims, so the exchange and the
+ * checking stay separable: the checking is pure and can be tested exhaustively
+ * without a network, which is where the security actually lives.
+ */
+export async function exchangeCode(config: GoogleConfig, code: string, codeVerifier: string): Promise<string> {
+  const response = await fetch(TOKEN_ENDPOINT, {
+    method: 'POST',
+    headers: { 'Content-Type': 'application/x-www-form-urlencoded' },
+    body: new URLSearchParams({
+      code,
+      client_id: config.clientId,
+      client_secret: config.clientSecret,
+      redirect_uri: config.redirectUri,
+      grant_type: 'authorization_code',
+      code_verifier: codeVerifier
+    })
+  });
+
+  if (!response.ok) {
+    // Logged rather than returned. The body names the client id and can carry
+    // the secret back in an error description, and none of it means anything to
+    // the customer.
+    const detail = await response.text().catch(() => '');
+    console.warn(`[google] token exchange failed: ${response.status} ${detail.slice(0, 300)}`);
+    throw new Error('the Google token exchange was refused');
+  }
+
+  const body = (await response.json()) as { id_token?: unknown };
+  if (typeof body.id_token !== 'string' || body.id_token === '') {
+    throw new Error('Google returned no id token');
+  }
+  return body.id_token;
+}
+
+/**
+ * The claims inside an id token, without verifying its signature.
+ *
+ * Named for what it does. Anywhere the token has not come straight back from
+ * the token endpoint over TLS, this function is the wrong one to call, and the
+ * name is meant to make that obvious at the call site.
+ */
+function decodeClaims(idToken: string): IdTokenClaims {
+  const [, payload] = idToken.split('.');
+  if (!payload) throw new Error('the id token is not a JWT');
+  try {
+    return JSON.parse(Buffer.from(payload, 'base64url').toString('utf8')) as IdTokenClaims;
+  } catch {
+    throw new Error('the id token payload is not JSON');
+  }
+}
+
+function asString(value: unknown): string | null {
+  return typeof value === 'string' && value !== '' ? value : null;
+}
+
+/**
+ * Who signed in, or a thrown error saying which check failed.
+ *
+ * Every check here is mandatory, and each one closes something specific:
+ *
+ * | Claim | What accepting it blindly would allow |
+ * | --- | --- |
+ * | `iss` | A token from an issuer we never chose to trust |
+ * | `aud` | A token minted for a different application, replayed here |
+ * | `exp` | A token captured once and reused indefinitely |
+ * | `nonce` | A token from an earlier attempt, replayed into this one |
+ * | `sub` | An identity row keyed on nothing |
+ *
+ * The messages name the failing check because they are logged, never shown. A
+ * customer sees one refusal for every cause, exactly as the passkey path does.
+ */
+export function verifiedIdentity(
+  idToken: string,
+  expected: { clientId: string; nonce: string },
+  now: Date = new Date()
+): GoogleIdentity {
+  const claims = decodeClaims(idToken);
+
+  if (typeof claims.iss !== 'string' || !ISSUERS.has(claims.iss)) {
+    throw new Error(`unexpected issuer: ${String(claims.iss)}`);
+  }
+  if (claims.aud !== expected.clientId) {
+    throw new Error('the id token was minted for a different client');
+  }
+
+  const exp = typeof claims.exp === 'number' ? claims.exp : NaN;
+  if (!Number.isFinite(exp)) throw new Error('the id token has no expiry');
+  if (exp + CLOCK_SKEW_SECONDS < Math.floor(now.getTime() / 1000)) {
+    throw new Error('the id token has expired');
+  }
+
+  // Compared in constant time. The nonce is a secret this server minted, and a
+  // byte-by-byte comparison that stops early is a timing oracle for it.
+  const nonce = asString(claims.nonce) ?? '';
+  const supplied = Buffer.from(nonce);
+  const wanted = Buffer.from(expected.nonce);
+  if (supplied.length !== wanted.length || !crypto.timingSafeEqual(supplied, wanted)) {
+    throw new Error('the id token belongs to a different sign-in attempt');
+  }
+
+  const sub = asString(claims.sub);
+  if (sub === null) throw new Error('the id token carries no subject');
+
+  const email = asString(claims.email);
+  if (email === null) throw new Error('the id token carries no email');
+
+  return {
+    sub,
+    email: email.toLowerCase().trim(),
+    // Strictly true, never merely truthy. Google sends a boolean, and treating
+    // the string "false" as a verified address is the exact mistake that turns
+    // the linking policy into an account-takeover path.
+    emailVerified: claims.email_verified === true,
+    firstName: asString(claims.given_name),
+    lastName: asString(claims.family_name)
+  };
+}
diff --git a/backend/src/google/returnTo.ts b/backend/src/google/returnTo.ts
new file mode 100644
index 0000000..d330ab8
--- /dev/null
+++ b/backend/src/google/returnTo.ts
@@ -0,0 +1,53 @@
+/**
+ * Where a customer is sent back to after signing in with Google (#341).
+ *
+ * An OAuth flow leaves this application entirely and comes back, so where the
+ * customer was has to survive the round trip — and the value doing that is one
+ * an attacker can propose, by handing somebody a link to our own start route
+ * with their destination attached.
+ *
+ * Unchecked, that makes the start route an open redirect wearing a sign-in flow
+ * as a disguise: a link on our real domain, with our real certificate, that
+ * deposits the customer somewhere else entirely. It is precisely the shape a
+ * credible phishing page wants, and it is worth more to an attacker than most
+ * bugs in the flow it hides behind.
+ *
+ * Its own module rather than a helper inside the route, so it can be tested
+ * without a database connection and so the next path that needs the same
+ * question has somewhere obvious to ask it.
+ */
+
+/** Where anyone goes when the answer is "not that". */
+export const DEFAULT_RETURN_TO = '/';
+
+/**
+ * A path inside this site, or the home page.
+ *
+ * Everything that is not plainly a local path is replaced rather than rejected.
+ * A refusal would mean a customer who signed in successfully sees an error
+ * about a query parameter they never typed, which helps nobody — the storefront
+ * is a fine place to land.
+ *
+ * The cases worth naming, because each is a way of writing "somewhere else"
+ * that still starts with a slash or looks like it might:
+ *
+ * - `//evil.test` is protocol-relative, and browsers treat it as absolute
+ * - `/\evil.test` is treated as protocol-relative by several browsers
+ * - `https://evil.test` does not start with a slash at all
+ * - a backslash anywhere in the authority position is normalised to a slash
+ */
+export function safeReturnTo(value: unknown): string {
+  if (typeof value !== 'string' || value === '') return DEFAULT_RETURN_TO;
+  if (!value.startsWith('/')) return DEFAULT_RETURN_TO;
+  // Both slashes, because browsers disagree about which they normalise.
+  if (value.startsWith('//') || value.startsWith('/\\')) return DEFAULT_RETURN_TO;
+  // A control character can truncate or split the Location header a browser
+  // reads. Checked by code point rather than by a regex, because a regex that
+  // matches control characters trips a lint rule existing for good reasons of
+  // its own, and this is clearer than an exemption from it.
+  for (const character of value) {
+    const code = character.codePointAt(0) ?? 0;
+    if (code < 0x20 || code === 0x7f) return DEFAULT_RETURN_TO;
+  }
+  return value;
+}
diff --git a/backend/src/rateLimit.ts b/backend/src/rateLimit.ts
index 4498cc9..2fdd1f6 100644
--- a/backend/src/rateLimit.ts
+++ b/backend/src/rateLimit.ts
@@ -184,3 +184,26 @@ export const intakeSubmitLimiter = rateLimit({
   legacyHeaders: false,
   message: { error: 'too many submissions — please try again later' }
 });
+
+/**
+ * Starting a Google sign-in (#341).
+ *
+ * The route mints three secrets and issues a redirect, which is cheap but not
+ * free, and it is reachable without a session by anyone who knows the URL.
+ *
+ * Generous, because a customer who bounces off Google's consent screen and
+ * tries again is doing something entirely reasonable and must never be told to
+ * wait. The limit exists so a loop cannot spend the server's entropy and fill
+ * the log, not to police customers.
+ *
+ * Keyed on the caller alone: this endpoint carries no email, which is the
+ * distinction the comment on the client-error limiter above draws.
+ */
+export const googleSignInLimiter = rateLimit({
+  windowMs: 15 * 60 * 1000,
+  limit: 60,
+  keyGenerator: keyByCaller,
+  standardHeaders: 'draft-7',
+  legacyHeaders: false,
+  message: { error: 'too many sign-in attempts — please try again shortly' }
+});
diff --git a/backend/src/routes/googleAuth.ts b/backend/src/routes/googleAuth.ts
new file mode 100644
index 0000000..0985f76
--- /dev/null
+++ b/backend/src/routes/googleAuth.ts
@@ -0,0 +1,213 @@
+import { Router, Request, Response } from 'express';
+import crypto from 'node:crypto';
+import { pool } from '../db';
+import { asyncRoute } from '../asyncRoute';
+import { signIn } from '../customerSession';
+import { googleConfig } from '../google/config';
+import { newAttempt, authorizationUrl, exchangeCode, verifiedIdentity } from '../google/oauth';
+import type { AttemptSecrets } from '../google/oauth';
+import { googleSignInLimiter } from '../rateLimit';
+import { safeReturnTo } from '../google/returnTo';
+
+const router = Router();
+
+/**
+ * Signing in with Google (#341).
+ *
+ * Unauthenticated by design — this is how a customer becomes authenticated —
+ * and mounted at `/api/auth/google`, away from `/api/customers`, because it is
+ * the first route in this application that a third party redirects into.
+ *
+ * ## What this phase does and does not do
+ *
+ * It signs in a customer whose Google identity is **already linked**. A
+ * successful sign-in by somebody with no identity row does nothing yet: account
+ * creation is #342 and the linking policy is #343, and holding them back keeps
+ * this change about the protocol alone.
+ *
+ * ## The cookie, and why it is the whole security of the callback
+ *
+ * The callback is a plain GET that anyone on the internet can invoke. What
+ * makes it safe is that it can only complete for a browser holding a cookie
+ * this server set moments earlier, carrying three secrets:
+ *
+ * - **state** proves the callback belongs to the request this browser started
+ * - **nonce** proves the id token was minted for this attempt
+ * - **code verifier** proves the code is being spent by whoever asked for it
+ *
+ * The cookie is cleared on every path through the callback, success or failure,
+ * so one attempt cannot be replayed even once.
+ */
+
+/** Ten minutes. Long enough to sign in, short enough that a stolen one is stale. */
+const ATTEMPT_TTL_MS = 10 * 60 * 1000;
+
+const ATTEMPT_COOKIE = 'rd_oauth';
+
+interface Attempt extends AttemptSecrets {
+  returnTo: string;
+}
+
+interface IdentityRow {
+  customer_id: number;
+  disabled_at: Date | null;
+}
+
+/**
+ * Where the customer is sent when this ends.
+ *
+ * Always a redirect, never JSON. The browser arrives here by following Google's
+ * redirect, so whatever this responds with is rendered as a page — and a bare
+ * JSON error is a dead end with no way back to the storefront.
+ */
+const FAILURE_PATH = '/login?auth=google-failed';
+
+function setAttemptCookie(res: Response, attempt: Attempt): void {
+  res.cookie(ATTEMPT_COOKIE, Buffer.from(JSON.stringify(attempt)).toString('base64url'), {
+    httpOnly: true,
+    secure: process.env.NODE_ENV === 'production',
+    // Lax, and NOT Strict. The callback arrives as a top-level navigation from
+    // Google, which is cross-site. Strict withholds the cookie, the state check
+    // then fails, and every sign-in is refused with an error that looks exactly
+    // like tampering. This one line is the single most expensive thing to get
+    // wrong in the whole flow.
+    sameSite: 'lax',
+    maxAge: ATTEMPT_TTL_MS,
+    path: '/'
+  });
+}
+
+function readAttemptCookie(req: Request): Attempt | null {
+  const raw = req.cookies?.[ATTEMPT_COOKIE];
+  if (typeof raw !== 'string' || raw === '') return null;
+  try {
+    const parsed = JSON.parse(Buffer.from(raw, 'base64url').toString('utf8')) as Partial<Attempt>;
+    if (
+      typeof parsed.state !== 'string' ||
+      typeof parsed.nonce !== 'string' ||
+      typeof parsed.codeVerifier !== 'string'
+    ) {
+      return null;
+    }
+    return {
+      state: parsed.state,
+      nonce: parsed.nonce,
+      codeVerifier: parsed.codeVerifier,
+      returnTo: typeof parsed.returnTo === 'string' ? parsed.returnTo : '/'
+    };
+  } catch {
+    return null;
+  }
+}
+
+/**
+ * Compares two secrets without leaking where they first differ.
+ *
+ * `timingSafeEqual` throws on buffers of unequal length, and a length check
+ * before it would leak the length, so both are hashed to a fixed 32 bytes
+ * first — the same trick `adminGate` uses, for the same reason.
+ */
+function secretsMatch(a: string, b: string): boolean {
+  const digest = (value: string) => crypto.createHash('sha256').update(value, 'utf8').digest();
+  return crypto.timingSafeEqual(digest(a), digest(b));
+}
+
+router.get(
+  '/start',
+  googleSignInLimiter,
+  asyncRoute(async (req: Request, res: Response) => {
+    const config = googleConfig();
+    if (!config.enabled) {
+      // Not a 404 and not an error page. Nothing offers this link when Google
+      // sign-in is switched off, so reaching it means a stale bookmark or a
+      // hand-typed URL, and the storefront is the right answer to both.
+      return res.redirect('/');
+    }
+
+    const attempt: Attempt = { ...newAttempt(), returnTo: safeReturnTo(req.query.returnTo) };
+    setAttemptCookie(res, attempt);
+    res.redirect(authorizationUrl(config, attempt));
+  })
+);
+
+router.get(
+  '/callback',
+  asyncRoute(async (req: Request, res: Response) => {
+    const config = googleConfig();
+    const attempt = readAttemptCookie(req);
+
+    // Cleared before anything is decided, on every path. A cookie that survives
+    // a failed attempt is a second try at the same state and nonce.
+    res.clearCookie(ATTEMPT_COOKIE, { path: '/' });
+
+    if (!config.enabled || attempt === null) return res.redirect(FAILURE_PATH);
+
+    // Google sends `error=access_denied` when the customer declines at the
+    // consent screen. That is a cancellation rather than a failure, and it goes
+    // back to the storefront with nothing said — the same distinction #41 draws
+    // for a dismissed passkey prompt.
+    if (typeof req.query.error === 'string') {
+      return res.redirect(attempt.returnTo);
+    }
+
+    const state = typeof req.query.state === 'string' ? req.query.state : '';
+    const code = typeof req.query.code === 'string' ? req.query.code : '';
+    if (state === '' || code === '' || !secretsMatch(state, attempt.state)) {
+      console.warn('[google] callback refused: state did not match the attempt cookie');
+      return res.redirect(FAILURE_PATH);
+    }
+
+    let identity;
+    try {
+      const idToken = await exchangeCode(config, code, attempt.codeVerifier);
+      identity = verifiedIdentity(idToken, { clientId: config.clientId, nonce: attempt.nonce });
+    } catch (err) {
+      // Logged, never returned. These messages name which check failed, which
+      // is exactly what the person reading the logs needs and exactly what an
+      // attacker would like to be told.
+      console.warn(`[google] callback refused: ${(err as Error).message}`);
+      return res.redirect(FAILURE_PATH);
+    }
+
+    const { rows } = await pool.query<IdentityRow>(
+      `SELECT i.customer_id, c.disabled_at
+       FROM customer_identities i
+       JOIN customers c ON c.id = i.customer_id
+       WHERE i.provider = 'google' AND i.provider_sub = $1`,
+      [identity.sub]
+    );
+    const linked = rows[0];
+
+    // No identity row means a customer this shop has never seen through Google.
+    // Creating one is #342 and linking to an existing account is #343; until
+    // those land there is nothing to do, and doing nothing must not look like a
+    // protocol failure.
+    if (!linked) return res.redirect(FAILURE_PATH);
+
+    // Refused here as well as on the password and passkey paths. Enforcing it
+    // on some routes and not others is how a disabled account keeps a way in,
+    // which is the reason #39 called this out for passkeys.
+    if (linked.disabled_at !== null) {
+      console.warn(`[google] refused a disabled account: customer ${linked.customer_id}`);
+      return res.redirect(FAILURE_PATH);
+    }
+
+    await pool.query(
+      `UPDATE customer_identities SET last_used_at = now()
+       WHERE provider = 'google' AND provider_sub = $1`,
+      [identity.sub]
+    );
+
+    // The same call password login and passkey login make. Not a third
+    // implementation that agrees today — the same one, so cookie flags, expiry
+    // and logout behave identically however a customer got here.
+    await signIn(res, linked.customer_id);
+
+    res.redirect(attempt.returnTo);
+  })
+);
+
+/** Exported for the tests; nothing else needs the cookie's name. */
+export { ATTEMPT_COOKIE, ATTEMPT_TTL_MS, FAILURE_PATH };
+
+export default router;
diff --git a/backend/tests/integration/googleSignIn.integration.test.ts b/backend/tests/integration/googleSignIn.integration.test.ts
new file mode 100644
index 0000000..023d2aa
--- /dev/null
+++ b/backend/tests/integration/googleSignIn.integration.test.ts
@@ -0,0 +1,356 @@
+import request from 'supertest';
+import app from '../../src/app';
+import { pool, requireRow } from '../../src/db';
+import { resetDb, closeDb } from './setup/testDb';
+
+const CLIENT_ID = 'test-client.apps.googleusercontent.com';
+const SUB = 'google-subject-1234567890';
+
+let fetchSpy: jest.SpyInstance;
+
+beforeEach(async () => {
+  await resetDb();
+  process.env.GOOGLE_CLIENT_ID = CLIENT_ID;
+  process.env.GOOGLE_CLIENT_SECRET = 'test-secret';
+  process.env.PUBLIC_URL = 'http://localhost:3000';
+  // Nothing here talks to Google. The exchange is the only network call in the
+  // flow, so stubbing it leaves every decision this suite cares about — the
+  // cookie, the state check, the claim checks, the lookup — running for real.
+  fetchSpy = jest.spyOn(globalThis, 'fetch');
+});
+
+afterEach(() => {
+  fetchSpy.mockRestore();
+  delete process.env.GOOGLE_CLIENT_ID;
+  delete process.env.GOOGLE_CLIENT_SECRET;
+  delete process.env.PUBLIC_URL;
+});
+
+afterAll(async () => {
+  await pool.end();
+  await closeDb();
+});
+
+/** An unsigned id token. Nothing in this flow reads a signature — see google/oauth.ts. */
+function idToken(claims: Record<string, unknown>): string {
+  const part = (value: unknown) => Buffer.from(JSON.stringify(value)).toString('base64url');
+  return `${part({ alg: 'RS256' })}.${part(claims)}.not-a-signature`;
+}
+
+function claimsFor(nonce: string, overrides: Record<string, unknown> = {}) {
+  return {
+    iss: 'https://accounts.google.com',
+    aud: CLIENT_ID,
+    exp: Math.floor(Date.now() / 1000) + 3600,
+    sub: SUB,
+    nonce,
+    email: 'customer@example.com',
+    email_verified: true,
+    given_name: 'Test',
+    family_name: 'Customer',
+    ...overrides
+  };
+}
+
+function respondWithToken(token: string): void {
+  fetchSpy.mockResolvedValue(
+    new Response(JSON.stringify({ id_token: token }), {
+      status: 200,
+      headers: { 'Content-Type': 'application/json' }
+    })
+  );
+}
+
+/** The attempt cookie the start route set, as a header for the callback. */
+function cookieHeader(setCookie: string[]): string {
+  const attempt = setCookie.find((c) => c.startsWith('rd_oauth='));
+  if (!attempt) throw new Error('the start route set no attempt cookie');
+  // Non-null: the header was just matched, so it has at least one segment.
+  return attempt.split(';')[0] as string;
+}
+
+/** One Set-Cookie value, as a request header. Throws rather than typing around its absence. */
+function requireCookie(setCookie: string[], prefix: string): string {
+  const found = setCookie.find((c) => c.startsWith(prefix));
+  if (!found) throw new Error(`no ${prefix} cookie was set`);
+  return found.split(';')[0] as string;
+}
+
+/** Reads the secrets back out of the cookie, which is the only place they exist. */
+function attemptFrom(setCookie: string[]): { state: string; nonce: string; returnTo: string } {
+  const value = cookieHeader(setCookie).slice('rd_oauth='.length);
+  return JSON.parse(Buffer.from(decodeURIComponent(value), 'base64url').toString('utf8'));
+}
+
+/** Starts a sign-in and hands back what the callback needs to finish it. */
+async function startSignIn(returnTo?: string) {
+  const res = await request(app)
+    .get('/api/auth/google/start')
+    .query(returnTo === undefined ? {} : { returnTo });
+  const setCookie = res.headers['set-cookie'] as unknown as string[];
+  return { res, cookie: cookieHeader(setCookie), ...attemptFrom(setCookie) };
+}
+
+async function createCustomer(email: string): Promise<number> {
+  const { rows } = await pool.query<{ id: number }>(
+    `INSERT INTO customers (email, password_hash, first_name, last_name, unsubscribe_token)
+     VALUES ($1, 'not-a-real-hash', 'Test', 'Customer', $2) RETURNING id`,
+    [email, `unsub-${email}`]
+  );
+  return requireRow(rows, 'the customer this test just created').id;
+}
+
+async function linkGoogle(customerId: number, sub = SUB): Promise<void> {
+  await pool.query(
+    `INSERT INTO customer_identities (customer_id, provider, provider_sub)
+     VALUES ($1, 'google', $2)`,
+    [customerId, sub]
+  );
+}
+
+describe('GET /api/auth/google/start', () => {
+  it('sends the customer to Google with the code flow and PKCE', async () => {
+    const { res } = await startSignIn();
+
+    expect(res.status).toBe(302);
+    const target = new URL(res.headers.location as string);
+    expect(target.origin).toBe('https://accounts.google.com');
+    expect(target.searchParams.get('response_type')).toBe('code');
+    expect(target.searchParams.get('code_challenge_method')).toBe('S256');
+  });
+
+  it('sets an httpOnly attempt cookie, which is the whole security of the callback', async () => {
+    const res = await request(app).get('/api/auth/google/start');
+    const attempt = (res.headers['set-cookie'] as unknown as string[]).find((c) =>
+      c.startsWith('rd_oauth=')
+    );
+
+    expect(attempt).toMatch(/HttpOnly/i);
+    // Lax and not Strict. Strict withholds the cookie on the cross-site
+    // top-level navigation back from Google, and every sign-in then fails the
+    // state check in a way that looks exactly like tampering.
+    expect(attempt).toMatch(/SameSite=Lax/i);
+  });
+
+  it('never puts the client secret anywhere the browser can see', async () => {
+    const res = await request(app).get('/api/auth/google/start');
+
+    expect(res.headers.location).not.toContain('test-secret');
+    expect(JSON.stringify(res.headers['set-cookie'])).not.toContain('test-secret');
+  });
+
+  it('carries a local return path through the round trip', async () => {
+    const { returnTo } = await startSignIn('/?max_price=50000');
+
+    expect(returnTo).toBe('/?max_price=50000');
+  });
+
+  it('refuses an off-site return path rather than becoming an open redirect', async () => {
+    const { returnTo } = await startSignIn('//evil.test');
+
+    expect(returnTo).toBe('/');
+  });
+
+  it('sends the customer to the storefront when Google sign-in is switched off', async () => {
+    delete process.env.GOOGLE_CLIENT_ID;
+    delete process.env.GOOGLE_CLIENT_SECRET;
+
+    const res = await request(app).get('/api/auth/google/start');
+
+    // A stale bookmark or a hand-typed URL, and the storefront answers both.
+    expect(res.status).toBe(302);
+    expect(res.headers.location).toBe('/');
+    expect(res.headers['set-cookie']).toBeUndefined();
+  });
+});
+
+describe('GET /api/auth/google/callback', () => {
+  it('signs in a customer whose Google identity is already linked', async () => {
+    const customerId = await createCustomer('linked@example.com');
+    await linkGoogle(customerId);
+    const { cookie, state, nonce } = await startSignIn('/cart');
+    respondWithToken(idToken(claimsFor(nonce)));
+
+    const res = await request(app)
+      .get('/api/auth/google/callback')
+      .query({ code: 'an-auth-code', state })
+      .set('Cookie', cookie);
+
+    expect(res.status).toBe(302);
+    expect(res.headers.location).toBe('/cart');
+    const session = (res.headers['set-cookie'] as unknown as string[]).find((c) =>
+      c.startsWith('rd_session=')
+    );
+    expect(session).toBeDefined();
+  });
+
+  it('produces a session the rest of the application accepts', async () => {
+    const customerId = await createCustomer('session@example.com');
+    await linkGoogle(customerId);
+    const { cookie, state, nonce } = await startSignIn();
+    respondWithToken(idToken(claimsFor(nonce)));
+
+    const res = await request(app)
+      .get('/api/auth/google/callback')
+      .query({ code: 'an-auth-code', state })
+      .set('Cookie', cookie);
+    const setCookie = res.headers['set-cookie'] as unknown as string[];
+    const session = requireCookie(setCookie, 'rd_session=');
+
+    // The point of sharing signIn with the password and passkey paths: the
+    // session is not merely present, it is the same kind of session.
+    const me = await request(app).get('/api/customers/me').set('Cookie', session);
+    expect(me.status).toBe(200);
+    expect(me.body.email).toBe('session@example.com');
+  });
+
+  it('stamps last_used_at, which is what tells two identities apart', async () => {
+    const customerId = await createCustomer('stamped@example.com');
+    await linkGoogle(customerId);
+    const { cookie, state, nonce } = await startSignIn();
+    respondWithToken(idToken(claimsFor(nonce)));
+
+    await request(app)
+      .get('/api/auth/google/callback')
+      .query({ code: 'an-auth-code', state })
+      .set('Cookie', cookie);
+
+    const { rows } = await pool.query<{ last_used_at: Date | null }>(
+      `SELECT last_used_at FROM customer_identities WHERE provider_sub = $1`,
+      [SUB]
+    );
+    expect(requireRow(rows, 'the identity just used').last_used_at).not.toBeNull();
+  });
+
+  it('clears the attempt cookie, so one attempt cannot be replayed', async () => {
+    const customerId = await createCustomer('once@example.com');
+    await linkGoogle(customerId);
+    const { cookie, state, nonce } = await startSignIn();
+    respondWithToken(idToken(claimsFor(nonce)));
+
+    const first = await request(app)
+      .get('/api/auth/google/callback')
+      .query({ code: 'an-auth-code', state })
+      .set('Cookie', cookie);
+
+    const cleared = (first.headers['set-cookie'] as unknown as string[]).find((c) =>
+      c.startsWith('rd_oauth=')
+    );
+    expect(cleared).toMatch(/rd_oauth=;/);
+  });
+
+  it('refuses a state that does not match the attempt cookie', async () => {
+    const { cookie } = await startSignIn();
+
+    const res = await request(app)
+      .get('/api/auth/google/callback')
+      .query({ code: 'an-auth-code', state: 'not-the-state-we-issued' })
+      .set('Cookie', cookie);
+
+    expect(res.headers.location).toBe('/login?auth=google-failed');
+    // Refused before any network call: a mismatched state is not worth a token
+    // exchange, and spending the code would be handing it to whoever forged it.
+    expect(fetchSpy).not.toHaveBeenCalled();
+  });
+
+  it('refuses a callback carrying no attempt cookie at all', async () => {
+    const res = await request(app)
+      .get('/api/auth/google/callback')
+      .query({ code: 'an-auth-code', state: 'anything' });
+
+    expect(res.headers.location).toBe('/login?auth=google-failed');
+    expect(fetchSpy).not.toHaveBeenCalled();
+  });
+
+  it('treats a declined consent screen as a cancellation, not a failure', async () => {
+    const { cookie } = await startSignIn('/cart');
+
+    const res = await request(app)
+      .get('/api/auth/google/callback')
+      .query({ error: 'access_denied' })
+      .set('Cookie', cookie);
+
+    // Back where they were, with nothing said. A customer who changed their
+    // mind has not encountered an error, which is the distinction #41 draws
+    // for a dismissed passkey prompt.
+    expect(res.headers.location).toBe('/cart');
+  });
+
+  it('refuses an id token minted for a different application', async () => {
+    const customerId = await createCustomer('wrongaud@example.com');
+    await linkGoogle(customerId);
+    const { cookie, state, nonce } = await startSignIn();
+    respondWithToken(idToken(claimsFor(nonce, { aud: 'someone-else.apps.googleusercontent.com' })));
+
+    const res = await request(app)
+      .get('/api/auth/google/callback')
+      .query({ code: 'an-auth-code', state })
+      .set('Cookie', cookie);
+
+    expect(res.headers.location).toBe('/login?auth=google-failed');
+    expect(res.headers['set-cookie'] ?? []).not.toContainEqual(expect.stringContaining('rd_session='));
+  });
+
+  it('refuses an id token from a different sign-in attempt', async () => {
+    const customerId = await createCustomer('wrongnonce@example.com');
+    await linkGoogle(customerId);
+    const { cookie, state } = await startSignIn();
+    respondWithToken(idToken(claimsFor('a-nonce-from-somewhere-else')));
+
+    const res = await request(app)
+      .get('/api/auth/google/callback')
+      .query({ code: 'an-auth-code', state })
+      .set('Cookie', cookie);
+
+    expect(res.headers.location).toBe('/login?auth=google-failed');
+  });
+
+  it('refuses when Google declines the token exchange', async () => {
+    const { cookie, state } = await startSignIn();
+    fetchSpy.mockResolvedValue(new Response('{"error":"invalid_grant"}', { status: 400 }));
+
+    const res = await request(app)
+      .get('/api/auth/google/callback')
+      .query({ code: 'a-spent-code', state })
+      .set('Cookie', cookie);
+
+    expect(res.headers.location).toBe('/login?auth=google-failed');
+  });
+
+  it('refuses a disabled account, as the password and passkey paths do', async () => {
+    const customerId = await createCustomer('disabled@example.com');
+    await linkGoogle(customerId);
+    await pool.query(`UPDATE customers SET disabled_at = now() WHERE id = $1`, [customerId]);
+    const { cookie, state, nonce } = await startSignIn();
+    respondWithToken(idToken(claimsFor(nonce)));
+
+    const res = await request(app)
+      .get('/api/auth/google/callback')
+      .query({ code: 'an-auth-code', state })
+      .set('Cookie', cookie);
+
+    // Enforcing it on some sign-in routes and not others is how a disabled
+    // account keeps a way in.
+    expect(res.headers.location).toBe('/login?auth=google-failed');
+    expect(res.headers['set-cookie'] ?? []).not.toContainEqual(expect.stringContaining('rd_session='));
+  });
+
+  it('does not sign in an unlinked Google account, and creates nothing', async () => {
+    await createCustomer('unlinked@example.com');
+    const { cookie, state, nonce } = await startSignIn();
+    respondWithToken(idToken(claimsFor(nonce, { email: 'unlinked@example.com' })));
+
+    const res = await request(app)
+      .get('/api/auth/google/callback')
+      .query({ code: 'an-auth-code', state })
+      .set('Cookie', cookie);
+
+    // Account creation is #342 and linking is #343. Matching on the address
+    // here would be the takeover path both of those exist to decide carefully.
+    expect(res.headers.location).toBe('/login?auth=google-failed');
+    const { rows } = await pool.query<{ n: number }>(
+      `SELECT count(*)::int AS n FROM customer_identities`
+    );
+    expect(requireRow(rows, 'a count of identities').n).toBe(0);
+  });
+});
diff --git a/backend/tests/unit/googleOauth.test.ts b/backend/tests/unit/googleOauth.test.ts
new file mode 100644
index 0000000..8a41b00
--- /dev/null
+++ b/backend/tests/unit/googleOauth.test.ts
@@ -0,0 +1,221 @@
+import crypto from 'node:crypto';
+import { authorizationUrl, codeChallenge, newAttempt, verifiedIdentity } from '../../src/google/oauth';
+import type { GoogleConfig } from '../../src/google/config';
+
+const CONFIG: GoogleConfig = {
+  clientId: 'id.apps.googleusercontent.com',
+  clientSecret: 'shh',
+  redirectUri: 'https://redefined-designs.com/api/auth/google/callback',
+  enabled: true
+};
+
+const NONCE = 'the-nonce-for-this-attempt';
+
+/** An id token with the given claims. Unsigned, because nothing here reads a signature. */
+function idToken(claims: Record<string, unknown>): string {
+  const part = (value: unknown) => Buffer.from(JSON.stringify(value)).toString('base64url');
+  return `${part({ alg: 'RS256' })}.${part(claims)}.not-a-signature`;
+}
+
+const VALID = {
+  iss: 'https://accounts.google.com',
+  aud: CONFIG.clientId,
+  exp: Math.floor(Date.now() / 1000) + 3600,
+  sub: '1234567890',
+  nonce: NONCE,
+  email: 'Customer@Example.com',
+  email_verified: true,
+  given_name: 'Test',
+  family_name: 'Customer'
+};
+
+/** VALID minus one claim, for the tests about a claim being absent. */
+function without(claim: keyof typeof VALID): Record<string, unknown> {
+  const copy: Record<string, unknown> = { ...VALID };
+  delete copy[claim];
+  return copy;
+}
+
+
+describe('authorizationUrl', () => {
+  const attempt = { state: 'st', nonce: 'no', codeVerifier: 'ver' };
+
+  it('sends the customer to Google with the code flow and PKCE', () => {
+    const url = new URL(authorizationUrl(CONFIG, attempt));
+
+    expect(url.origin + url.pathname).toBe('https://accounts.google.com/o/oauth2/v2/auth');
+    expect(url.searchParams.get('response_type')).toBe('code');
+    expect(url.searchParams.get('client_id')).toBe(CONFIG.clientId);
+    expect(url.searchParams.get('redirect_uri')).toBe(CONFIG.redirectUri);
+    expect(url.searchParams.get('state')).toBe('st');
+    expect(url.searchParams.get('nonce')).toBe('no');
+    expect(url.searchParams.get('code_challenge_method')).toBe('S256');
+    expect(url.searchParams.get('code_challenge')).toBe(codeChallenge('ver'));
+  });
+
+  it('asks for exactly the three non-sensitive scopes', () => {
+    // Anything beyond these turns publishing into a verification review with a
+    // video walkthrough and a wait measured in weeks.
+    const url = new URL(authorizationUrl(CONFIG, attempt));
+
+    expect(url.searchParams.get('scope')?.split(' ').sort()).toEqual(['email', 'openid', 'profile']);
+  });
+
+  it('never asks for offline access', () => {
+    // A refresh token would be a long-lived credential with nothing to spend it
+    // on. Google issues one only when asked, so the check is that we do not ask.
+    const url = new URL(authorizationUrl(CONFIG, attempt));
+
+    expect(url.searchParams.get('access_type')).toBeNull();
+    expect(url.searchParams.get('prompt')).toBeNull();
+  });
+
+  it('never puts the client secret in a URL the browser follows', () => {
+    expect(authorizationUrl(CONFIG, attempt)).not.toContain(CONFIG.clientSecret);
+  });
+});
+
+describe('newAttempt', () => {
+  it('mints three different secrets', () => {
+    // Three rather than one reused: they are checked by different parties at
+    // different moments, and a single value would mean anything learning it
+    // from one check satisfies the others.
+    const { state, nonce, codeVerifier } = newAttempt();
+
+    expect(new Set([state, nonce, codeVerifier]).size).toBe(3);
+  });
+
+  it('does not repeat itself', () => {
+    expect(newAttempt().state).not.toBe(newAttempt().state);
+  });
+});
+
+describe('codeChallenge', () => {
+  it('is the base64url SHA-256 of the verifier, which is what S256 means', () => {
+    const verifier = 'a-verifier';
+
+    expect(codeChallenge(verifier)).toBe(
+      crypto.createHash('sha256').update(verifier).digest('base64url')
+    );
+  });
+});
+
+/**
+ * The security of the whole flow lives here.
+ *
+ * No signature is verified, because the token arrives on a direct TLS
+ * connection to Google's token endpoint — the case OpenID Connect explicitly
+ * permits skipping it. That makes every one of these claim checks load-bearing
+ * rather than belt-and-braces, so each has a test naming what accepting it
+ * blindly would allow.
+ */
+describe('verifiedIdentity', () => {
+  const expected = { clientId: CONFIG.clientId, nonce: NONCE };
+
+  it('accepts a well-formed token and reports who signed in', () => {
+    const identity = verifiedIdentity(idToken(VALID), expected);
+
+    expect(identity.sub).toBe('1234567890');
+    expect(identity.emailVerified).toBe(true);
+    expect(identity.firstName).toBe('Test');
+    expect(identity.lastName).toBe('Customer');
+  });
+
+  it('normalises the email the way registration does', () => {
+    // A stricter comparison than registration's would silently fail to match an
+    // existing customer and produce a duplicate account instead (#343).
+    expect(verifiedIdentity(idToken(VALID), expected).email).toBe('customer@example.com');
+  });
+
+  it('accepts both spellings of the issuer, because Google sends both', () => {
+    // Accepting only one produces sign-ins that fail for some customers and not
+    // others, which is about the least diagnosable failure this flow can have.
+    for (const iss of ['https://accounts.google.com', 'accounts.google.com']) {
+      expect(verifiedIdentity(idToken({ ...VALID, iss }), expected).sub).toBe('1234567890');
+    }
+  });
+
+  it('refuses an issuer it was not told to trust', () => {
+    expect(() => verifiedIdentity(idToken({ ...VALID, iss: 'https://evil.test' }), expected)).toThrow(
+      /unexpected issuer/
+    );
+  });
+
+  it('refuses a token minted for a different application', () => {
+    // Without this, a token obtained by any other Google app could be replayed
+    // here and would sign somebody in.
+    expect(() =>
+      verifiedIdentity(idToken({ ...VALID, aud: 'someone-else.apps.googleusercontent.com' }), expected)
+    ).toThrow(/different client/);
+  });
+
+  it('refuses an expired token', () => {
+    const expired = { ...VALID, exp: Math.floor(Date.now() / 1000) - 3600 };
+
+    expect(() => verifiedIdentity(idToken(expired), expected)).toThrow(/expired/);
+  });
+
+  it('allows a minute of clock skew, so a healthy host does not refuse valid tokens', () => {
+    const justExpired = { ...VALID, exp: Math.floor(Date.now() / 1000) - 5 };
+
+    expect(verifiedIdentity(idToken(justExpired), expected).sub).toBe('1234567890');
+  });
+
+  it('refuses a token with no expiry at all', () => {
+    expect(() => verifiedIdentity(idToken(without('exp')), expected)).toThrow(/no expiry/);
+  });
+
+  it('refuses a token from a different sign-in attempt', () => {
+    // This is what stops a token captured from one attempt being replayed into
+    // another, which is the whole reason the nonce exists.
+    expect(() => verifiedIdentity(idToken({ ...VALID, nonce: 'someone-elses' }), expected)).toThrow(
+      /different sign-in attempt/
+    );
+  });
+
+  it('refuses a token carrying no nonce', () => {
+    expect(() => verifiedIdentity(idToken(without('nonce')), expected)).toThrow(
+      /different sign-in attempt/
+    );
+  });
+
+  it('refuses a token with no subject, which is the identity itself', () => {
+    expect(() => verifiedIdentity(idToken(without('sub')), expected)).toThrow(/no subject/);
+  });
+
+  it('refuses a token with no email', () => {
+    expect(() => verifiedIdentity(idToken(without('email')), expected)).toThrow(/no email/);
+  });
+
+  describe('email_verified', () => {
+    it('is true only for the boolean, never a truthy string', () => {
+      // The linking policy turns entirely on this flag (#343). Treating the
+      // string "false" as verified is exactly the mistake that would make
+      // auto-linking an account-takeover path.
+      expect(verifiedIdentity(idToken({ ...VALID, email_verified: 'false' }), expected).emailVerified)
+        .toBe(false);
+      expect(verifiedIdentity(idToken({ ...VALID, email_verified: 'true' }), expected).emailVerified)
+        .toBe(false);
+    });
+
+    it('is false when the claim is missing', () => {
+      expect(verifiedIdentity(idToken(without('email_verified')), expected).emailVerified).toBe(false);
+    });
+  });
+
+  it('tolerates a profile with no names, because Google may send none', () => {
+    const anonymous = { ...without('given_name') };
+    delete anonymous.family_name;
+    const identity = verifiedIdentity(idToken(anonymous), expected);
+
+    expect(identity.firstName).toBeNull();
+    expect(identity.lastName).toBeNull();
+  });
+
+  it.each([
+    ['not a JWT', 'nonsense'],
+    ['a JWT whose payload is not JSON', 'aGVhZGVy.bm90LWpzb24.sig']
+  ])('refuses %s', (_label, token) => {
+    expect(() => verifiedIdentity(token, expected)).toThrow();
+  });
+});
diff --git a/backend/tests/unit/googleReturnTo.test.ts b/backend/tests/unit/googleReturnTo.test.ts
new file mode 100644
index 0000000..c20fa5b
Binary files /dev/null and b/backend/tests/unit/googleReturnTo.test.ts differ