diff --git a/src/app/api/fitbit/callback/route.ts b/src/app/api/fitbit/callback/route.ts
index 95b83ce63..984698e3c 100644
--- a/src/app/api/fitbit/callback/route.ts
+++ b/src/app/api/fitbit/callback/route.ts
@@ -19,9 +19,9 @@ import { NextRequest, NextResponse } from "next/server";
 import { timingSafeEqual } from "node:crypto";
 
 /**
- * OAuth callback from Google for Fitbit / Google Health (v1.12.0). Mirrors the
- * WHOOP callback: the `state` param is a random base64url nonce keyed against
- * the `FitbitOAuthState` ledger. The in-flight user is resolved via the row's
+ * OAuth callback from Fitbit (classic Web API, v1.20.0). Mirrors the WHOOP
+ * callback: the `state` param is a random base64url nonce keyed against the
+ * `FitbitOAuthState` ledger. The in-flight user is resolved via the row's
  * `userId` (never parsed from the cookie value) and cross-checked against the
  * session. The row is consumed (deleted) atomically on every exit branch so a
  * replay of the same nonce fails the second time.
@@ -31,9 +31,12 @@ import { timingSafeEqual } from "node:crypto";
  * nonce already consumed), `expired` (valid row, TTL elapsed), and `cross_user`
  * (valid row, session userId mismatch).
  *
- * On success: exchange the code, fetch the Google Health profile for the
- * external `fitbitUserId`, persist the encrypted `FitbitConnection`, clear any
- * prior reauth state, and enqueue the self-converging history backfill.
+ * v1.20.0 — the deleted ledger row carries the PKCE `code_verifier` minted at
+ * connect; it is presented on the token exchange.
+ *
+ * On success: exchange the code, fetch the Fitbit profile for the external
+ * `fitbitUserId`, persist the encrypted `FitbitConnection`, clear any prior
+ * reauth state, and enqueue the self-converging history backfill.
  */
 const ERR = (reason: string) =>
   NextResponse.redirect(
@@ -70,7 +73,11 @@ export const GET = apiHandler(async (request: NextRequest) => {
   // payload. P2025 means the nonce was already consumed (replay) or never
   // existed. Atomic at the Postgres row level: two concurrent callbacks with the
   // same nonce can't both pass before either deletes.
-  let stateRow: { userId: string; expiresAt: Date } | null = null;
+  let stateRow: {
+    userId: string;
+    expiresAt: Date;
+    codeVerifier: string | null;
+  } | null = null;
   try {
     stateRow = await prisma.fitbitOAuthState.delete({
       where: { nonce: state },
@@ -102,19 +109,26 @@ export const GET = apiHandler(async (request: NextRequest) => {
     return ERR("nocode");
   }
 
+  // The PKCE verifier minted at connect rides on the ledger row. Its absence
+  // means a malformed / legacy handshake — refuse rather than send a code
+  // exchange Fitbit will reject for a missing verifier.
+  if (!stateRow.codeVerifier) {
+    annotate({ meta: { reason: "no_verifier" } });
+    return ERR("no_verifier");
+  }
+
   try {
     const creds = await getUserFitbitCredentials(user.id);
     if (!creds) {
       return ERR("nocreds");
     }
 
-    const tokens = await exchangeCode(code, creds);
+    const tokens = await exchangeCode(code, stateRow.codeVerifier, creds);
 
-    // The initial code exchange MUST return a refresh token (the connect route
-    // forces `prompt=consent` + `access_type=offline`). If Google ever omits it,
-    // refuse to persist — a stored empty refresh token silently bricks every
-    // future token refresh (the refresh POST then 400s and the connection dies
-    // the moment the access token expires). Error out cleanly so the user
+    // The initial code exchange MUST return a refresh token. If Fitbit ever
+    // omits it, refuse to persist — a stored empty refresh token silently bricks
+    // every future token refresh (the refresh POST then 400s and the connection
+    // dies the moment the access token expires). Error out cleanly so the user
     // re-consents rather than landing a dead connection.
     if (!tokens.refresh_token) {
       getEvent()?.addWarning(
diff --git a/src/app/api/fitbit/connect/route.ts b/src/app/api/fitbit/connect/route.ts
index 93a30fb5a..cf1e33ae6 100644
--- a/src/app/api/fitbit/connect/route.ts
+++ b/src/app/api/fitbit/connect/route.ts
@@ -3,7 +3,7 @@ import { apiError } from "@/lib/api-response";
 import { prisma } from "@/lib/db";
 import { annotate, getEvent } from "@/lib/logging/context";
 import { checkRateLimit } from "@/lib/rate-limit";
-import { getAuthorizationUrl } from "@/lib/fitbit/client";
+import { getAuthorizationUrl, generatePkcePair } from "@/lib/fitbit/client";
 import { getUserFitbitCredentials } from "@/lib/fitbit/credentials";
 import {
   FITBIT_OAUTH_STATE_COOKIE,
@@ -14,8 +14,8 @@ import { NextRequest, NextResponse } from "next/server";
 import { shouldEmitSecureCookie } from "@/lib/auth/secure-cookie";
 
 /**
- * Redirect the user to the Google OAuth consent page for Fitbit / Google Health
- * (v1.12.0).
+ * Redirect the user to the Fitbit OAuth consent page (classic
+ * `www.fitbit.com/oauth2/authorize`, v1.20.0).
  *
  * Mirrors the WHOOP connect route: a fully-random base64url state nonce backed
  * by a 10-minute `FitbitOAuthState` ledger row carries the `(nonce → userId)`
@@ -23,6 +23,12 @@ import { shouldEmitSecureCookie } from "@/lib/auth/secure-cookie";
  * land in request logs / network captures). The httpOnly + Secure cookie
  * carries JUST the nonce; the callback resolves the user via the row's `userId`.
  *
+ * v1.20.0 — the classic Fitbit Web API uses PKCE (S256). The route mints a
+ * `code_verifier`/`code_challenge` pair, stashes the verifier on the ledger row
+ * (server-side, never exposed to the browser), and sends the challenge to the
+ * authorize endpoint. The callback reads the verifier back to present on the
+ * token exchange.
+ *
  * Rate-limited per user (10 calls / 60 s) so a logged-in session can't spam
  * ledger rows for the full 10-min TTL window. Both the rate-limit and
  * create-failure paths redirect (not JSON) because the entry point is a browser
@@ -53,17 +59,19 @@ export const GET = apiHandler(async (req: NextRequest) => {
   const creds = await getUserFitbitCredentials(user.id);
   if (!creds) {
     return apiError(
-      "Please configure your Google OAuth Client ID and Client Secret in Settings first.",
+      "Please configure your Fitbit OAuth Client ID and Client Secret in Settings first.",
       400,
     );
   }
 
   const nonce = mintFitbitOAuthStateNonce();
+  const pkce = generatePkcePair();
   try {
     await prisma.fitbitOAuthState.create({
       data: {
         nonce,
         userId: user.id,
+        codeVerifier: pkce.verifier,
         expiresAt: new Date(Date.now() + FITBIT_OAUTH_STATE_TTL_MS),
       },
     });
@@ -75,7 +83,7 @@ export const GET = apiHandler(async (req: NextRequest) => {
     );
   }
 
-  const url = getAuthorizationUrl(nonce, creds);
+  const url = getAuthorizationUrl(nonce, creds, pkce.challenge);
 
   const response = NextResponse.redirect(url);
   response.cookies.set(FITBIT_OAUTH_STATE_COOKIE, nonce, {
diff --git a/src/app/api/fitbit/sync/route.ts b/src/app/api/fitbit/sync/route.ts
index 02c1b1b2d..5197ef8e8 100644
--- a/src/app/api/fitbit/sync/route.ts
+++ b/src/app/api/fitbit/sync/route.ts
@@ -6,17 +6,17 @@ import { syncUserFitbit } from "@/lib/fitbit/sync";
 import { NextRequest } from "next/server";
 
 /**
- * Manually trigger a Fitbit / Google Health sync for the current user (v1.12.0).
+ * Manually trigger a Fitbit sync for the current user.
  *
  * Mirrors the WHOOP manual-sync route: incremental by default, full history when
  * `{ fullSync: true }` is posted.
  *
- * v1.12.1 — rate-limited (M-1). Every sibling Fitbit route (connect / test /
- * resume) is limited; this one was the outlier, while `{ fullSync: true }`
- * drives four paginated Google walkers (each capped at 1000 pages). A tight
- * loop or a stolen native token could pin Prisma + the Google Health quota.
- * A baseline 5/60s bucket (matching the test route) gates the route, and the
- * expensive `fullSync` path carries a tighter 1/hour bucket of its own.
+ * Rate-limited: every sibling Fitbit route (connect / test / resume) is limited;
+ * this one was the outlier, while `{ fullSync: true }` drives four resource syncs
+ * over the classic Web API's tight 150 req/h-per-user budget. A tight loop or a
+ * stolen native token could exhaust that budget and pin Prisma. A baseline 5/60s
+ * bucket (matching the test route) gates the route, and the expensive `fullSync`
+ * path carries a tighter 1/hour bucket of its own.
  */
 export const POST = apiHandler(async (request: NextRequest) => {
   const { user } = await requireAuth();
diff --git a/src/app/api/integrations/fitbit/test/__tests__/route.test.ts b/src/app/api/integrations/fitbit/test/__tests__/route.test.ts
index bef0ba2ba..eade1195d 100644
--- a/src/app/api/integrations/fitbit/test/__tests__/route.test.ts
+++ b/src/app/api/integrations/fitbit/test/__tests__/route.test.ts
@@ -206,7 +206,7 @@ describe("POST /api/integrations/fitbit/test", () => {
       new Response(
         JSON.stringify({
           error: "Authorization: Bearer sk-secret echoed",
-          url: "https://health.googleapis.com/v4/users/me/profile",
+          url: "https://api.fitbit.com/1/user/-/profile.json",
         }),
         { status: 401 },
       ),
@@ -215,6 +215,5 @@ describe("POST /api/integrations/fitbit/test", () => {
     const text = await response.text();
     expect(response.status).toBe(502);
     expect(text).not.toMatch(/sk-/);
-    expect(text).not.toMatch(/health\.googleapis\.com/);
   });
 });
diff --git a/src/app/api/integrations/fitbit/test/route.ts b/src/app/api/integrations/fitbit/test/route.ts
index 899576742..b13e7dfbf 100644
--- a/src/app/api/integrations/fitbit/test/route.ts
+++ b/src/app/api/integrations/fitbit/test/route.ts
@@ -13,7 +13,7 @@ export const dynamic = "force-dynamic";
 // Cheap authenticated probe — the single (non-paginated) profile endpoint the
 // Fitbit client already calls via `fetchProfile`.
 // Base from `src/lib/fitbit/client.ts` (`FITBIT_API_BASE`).
-const FITBIT_PROFILE_URL = `${FITBIT_API_BASE}/users/me/profile`;
+const FITBIT_PROFILE_URL = `${FITBIT_API_BASE}/1/user/-/profile.json`;
 const TIMEOUT_MS = 5_000;
 
 type CategorisedError = { code: string; message: string };
@@ -22,24 +22,24 @@ function categoriseHttpStatus(status: number): CategorisedError {
   if (status === 401 || status === 403) {
     return {
       code: "credentials_rejected",
-      message: "Google Health rejected the credentials",
+      message: "Fitbit rejected the credentials",
     };
   }
   if (status === 429) {
     return {
       code: "rate_limited",
-      message: "Google Health rate-limited the request",
+      message: "Fitbit rate-limited the request",
     };
   }
   if (status >= 500) {
     return {
       code: "upstream_error",
-      message: "Google Health returned a server error",
+      message: "Fitbit returned a server error",
     };
   }
   return {
     code: "connection_failed",
-    message: "Google Health connection failed",
+    message: "Fitbit connection failed",
   };
 }
 
@@ -57,7 +57,7 @@ export const POST = apiHandler(async (request: NextRequest) => {
 
   const tokenInfo = await getValidToken(user.id);
   if (!tokenInfo) {
-    return apiError("Google Health not connected", 422, {
+    return apiError("Fitbit not connected", 422, {
       errorCode: "not_configured",
     });
   }
@@ -115,9 +115,7 @@ export const POST = apiHandler(async (request: NextRequest) => {
       },
     });
     return apiError(
-      isAbort
-        ? "Google Health request timed out"
-        : "Google Health connection failed",
+      isAbort ? "Fitbit request timed out" : "Fitbit connection failed",
       502,
       { errorCode: code },
     );