diff --git a/CHANGELOG.md b/CHANGELOG.md
index 1ab67237668e..b737ba882ce1 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,6 +4,10 @@
 
 - "You miss 100 percent of the chances you don't take. — Wayne Gretzky" — Michael Scott
 
+- **test(tanstackstart-react): Move initialization to client entry point ([#21161](https://github.com/getsentry/sentry-javascript/pull/21161))**
+
+  Change the recommended setup for the SDK to do `Sentry.init()` in the client entry file to capture telemetry that is emitted ahead of page hydration.
+
 ## 10.54.0
 
 ### Important Changes
diff --git a/dev-packages/e2e-tests/test-applications/tanstackstart-react-cloudflare/src/client.tsx b/dev-packages/e2e-tests/test-applications/tanstackstart-react-cloudflare/src/client.tsx
new file mode 100644
index 000000000000..5cc8b2a9c7f7
--- /dev/null
+++ b/dev-packages/e2e-tests/test-applications/tanstackstart-react-cloudflare/src/client.tsx
@@ -0,0 +1,27 @@
+import * as Sentry from '@sentry/browser';
+import { StartClient } from '@tanstack/react-start/client';
+import { StrictMode, startTransition } from 'react';
+import { hydrateRoot } from 'react-dom/client';
+
+Sentry.init({
+  environment: 'qa',
+  dsn: 'https://public@dsn.ingest.sentry.io/1337',
+  tracesSampleRate: 1.0,
+  release: 'e2e-test',
+  tunnel: 'http://localhost:3031/',
+});
+
+console.log('early-breadcrumb-from-client-entry');
+
+if (window.location.pathname === '/crash-before-hydration') {
+  throw new Error('Client Entry Crash');
+}
+
+startTransition(() => {
+  hydrateRoot(
+    document,
+    <StrictMode>
+      <StartClient />
+    </StrictMode>,
+  );
+});
diff --git a/dev-packages/e2e-tests/test-applications/tanstackstart-react-cloudflare/src/router.tsx b/dev-packages/e2e-tests/test-applications/tanstackstart-react-cloudflare/src/router.tsx
index c18a1a0b8167..386bebc8c363 100644
--- a/dev-packages/e2e-tests/test-applications/tanstackstart-react-cloudflare/src/router.tsx
+++ b/dev-packages/e2e-tests/test-applications/tanstackstart-react-cloudflare/src/router.tsx
@@ -9,14 +9,7 @@ export const getRouter = () => {
   });
 
   if (!router.isServer) {
-    Sentry.init({
-      environment: 'qa',
-      dsn: 'https://public@dsn.ingest.sentry.io/1337',
-      integrations: [Sentry.browserTracingIntegration()],
-      tracesSampleRate: 1.0,
-      release: 'e2e-test',
-      tunnel: 'http://localhost:3031/',
-    });
+    Sentry.addIntegration(Sentry.browserTracingIntegration());
   }
 
   return router;
diff --git a/dev-packages/e2e-tests/test-applications/tanstackstart-react-cloudflare/src/routes/crash-before-hydration.tsx b/dev-packages/e2e-tests/test-applications/tanstackstart-react-cloudflare/src/routes/crash-before-hydration.tsx
new file mode 100644
index 000000000000..772a045a7e33
--- /dev/null
+++ b/dev-packages/e2e-tests/test-applications/tanstackstart-react-cloudflare/src/routes/crash-before-hydration.tsx
@@ -0,0 +1,13 @@
+import { createFileRoute } from '@tanstack/react-router';
+
+export const Route = createFileRoute('/crash-before-hydration')({
+  component: CrashPage,
+});
+
+function CrashPage() {
+  return (
+    <div>
+      <p>This page crashes in client.tsx before hydration</p>
+    </div>
+  );
+}
diff --git a/dev-packages/e2e-tests/test-applications/tanstackstart-react-cloudflare/tests/early-init.test.ts b/dev-packages/e2e-tests/test-applications/tanstackstart-react-cloudflare/tests/early-init.test.ts
new file mode 100644
index 000000000000..2b58f0bf594e
--- /dev/null
+++ b/dev-packages/e2e-tests/test-applications/tanstackstart-react-cloudflare/tests/early-init.test.ts
@@ -0,0 +1,23 @@
+import { expect, test } from '@playwright/test';
+import { waitForError } from '@sentry-internal/test-utils';
+
+test('should capture errors thrown in client entry before hydration', async ({ page }) => {
+  const errorEventPromise = waitForError('tanstackstart-react-cloudflare', errorEvent => {
+    return errorEvent?.exception?.values?.[0]?.value === 'Client Entry Crash';
+  });
+
+  await page.goto('/crash-before-hydration');
+
+  const errorEvent = await errorEventPromise;
+
+  expect(errorEvent.exception?.values?.[0]).toMatchObject({
+    type: 'Error',
+    value: 'Client Entry Crash',
+  });
+
+  const consoleBreadcrumbs = errorEvent.breadcrumbs?.filter(
+    b => b.category === 'console' && b.message?.includes('early-breadcrumb-from-client-entry'),
+  );
+
+  expect(consoleBreadcrumbs?.length).toBeGreaterThanOrEqual(1);
+});
diff --git a/dev-packages/e2e-tests/test-applications/tanstackstart-react/src/client.tsx b/dev-packages/e2e-tests/test-applications/tanstackstart-react/src/client.tsx
new file mode 100644
index 000000000000..318bfbdf2285
--- /dev/null
+++ b/dev-packages/e2e-tests/test-applications/tanstackstart-react/src/client.tsx
@@ -0,0 +1,29 @@
+import * as Sentry from '@sentry/tanstackstart-react';
+import { StartClient } from '@tanstack/react-start/client';
+import { StrictMode, startTransition } from 'react';
+import { hydrateRoot } from 'react-dom/client';
+
+Sentry.init({
+  environment: 'qa', // dynamic sampling bias to keep transactions
+  dsn: __APP_DSN__,
+  // We recommend adjusting this value in production, or using tracesSampler
+  // for finer control
+  tracesSampleRate: 1.0,
+  release: 'e2e-test',
+  tunnel: __APP_TUNNEL__,
+});
+
+console.log('early-breadcrumb-from-client-entry');
+
+if (window.location.pathname === '/crash-before-hydration') {
+  throw new Error('Client Entry Crash');
+}
+
+startTransition(() => {
+  hydrateRoot(
+    document,
+    <StrictMode>
+      <StartClient />
+    </StrictMode>,
+  );
+});
diff --git a/dev-packages/e2e-tests/test-applications/tanstackstart-react/src/router.tsx b/dev-packages/e2e-tests/test-applications/tanstackstart-react/src/router.tsx
index 9a39b6f35c42..670b22bc688d 100644
--- a/dev-packages/e2e-tests/test-applications/tanstackstart-react/src/router.tsx
+++ b/dev-packages/e2e-tests/test-applications/tanstackstart-react/src/router.tsx
@@ -9,16 +9,7 @@ export const getRouter = () => {
   });
 
   if (!router.isServer) {
-    Sentry.init({
-      environment: 'qa', // dynamic sampling bias to keep transactions
-      dsn: __APP_DSN__,
-      integrations: [Sentry.tanstackRouterBrowserTracingIntegration(router)],
-      // We recommend adjusting this value in production, or using tracesSampler
-      // for finer control
-      tracesSampleRate: 1.0,
-      release: 'e2e-test',
-      tunnel: __APP_TUNNEL__,
-    });
+    Sentry.addIntegration(Sentry.tanstackRouterBrowserTracingIntegration(router));
   }
 
   return router;
diff --git a/dev-packages/e2e-tests/test-applications/tanstackstart-react/src/routes/crash-before-hydration.tsx b/dev-packages/e2e-tests/test-applications/tanstackstart-react/src/routes/crash-before-hydration.tsx
new file mode 100644
index 000000000000..772a045a7e33
--- /dev/null
+++ b/dev-packages/e2e-tests/test-applications/tanstackstart-react/src/routes/crash-before-hydration.tsx
@@ -0,0 +1,13 @@
+import { createFileRoute } from '@tanstack/react-router';
+
+export const Route = createFileRoute('/crash-before-hydration')({
+  component: CrashPage,
+});
+
+function CrashPage() {
+  return (
+    <div>
+      <p>This page crashes in client.tsx before hydration</p>
+    </div>
+  );
+}
diff --git a/dev-packages/e2e-tests/test-applications/tanstackstart-react/tests/early-init.test.ts b/dev-packages/e2e-tests/test-applications/tanstackstart-react/tests/early-init.test.ts
new file mode 100644
index 000000000000..ca2c7e44ee14
--- /dev/null
+++ b/dev-packages/e2e-tests/test-applications/tanstackstart-react/tests/early-init.test.ts
@@ -0,0 +1,28 @@
+import { expect, test } from '@playwright/test';
+import { waitForError } from '@sentry-internal/test-utils';
+
+const usesManagedTunnelRoute =
+  (process.env.E2E_TEST_TUNNEL_ROUTE_MODE ?? 'off') !== 'off' || process.env.E2E_TEST_CUSTOM_TUNNEL_ROUTE === '1';
+
+test.skip(usesManagedTunnelRoute, 'Default e2e suites run only in the proxy variant');
+
+test('should capture errors thrown in client entry before hydration', async ({ page }) => {
+  const errorEventPromise = waitForError('tanstackstart-react', errorEvent => {
+    return errorEvent?.exception?.values?.[0]?.value === 'Client Entry Crash';
+  });
+
+  await page.goto('/crash-before-hydration');
+
+  const errorEvent = await errorEventPromise;
+
+  expect(errorEvent.exception?.values?.[0]).toMatchObject({
+    type: 'Error',
+    value: 'Client Entry Crash',
+  });
+
+  const consoleBreadcrumbs = errorEvent.breadcrumbs?.filter(
+    b => b.category === 'console' && b.message?.includes('early-breadcrumb-from-client-entry'),
+  );
+
+  expect(consoleBreadcrumbs?.length).toBeGreaterThanOrEqual(1);
+});