diff --git a/dev-packages/e2e-tests/test-applications/tanstack-router/package.json b/dev-packages/e2e-tests/test-applications/tanstack-router/package.json
index 65086e5b4953..9e9f8660a899 100644
--- a/dev-packages/e2e-tests/test-applications/tanstack-router/package.json
+++ b/dev-packages/e2e-tests/test-applications/tanstack-router/package.json
@@ -9,7 +9,9 @@
     "test": "playwright test",
     "clean": "npx rimraf node_modules pnpm-lock.yaml",
     "test:build": "pnpm install && pnpm build",
-    "test:assert": "pnpm test"
+    "test:assert": "pnpm test",
+    "test:build:basepath": "E2E_TEST_BASEPATH=/app pnpm test:build",
+    "test:assert:basepath": "E2E_TEST_BASEPATH=/app pnpm test:assert"
   },
   "dependencies": {
     "@sentry/react": "file:../../packed/sentry-react-packed.tgz",
@@ -30,5 +32,14 @@
   },
   "volta": {
     "extends": "../../package.json"
+  },
+  "sentryTest": {
+    "variants": [
+      {
+        "build-command": "pnpm test:build:basepath",
+        "assert-command": "pnpm test:assert:basepath",
+        "label": "tanstack-router (basepath)"
+      }
+    ]
   }
 }
diff --git a/dev-packages/e2e-tests/test-applications/tanstack-router/src/main.tsx b/dev-packages/e2e-tests/test-applications/tanstack-router/src/main.tsx
index 3c2ed2905383..de4fca6ff30a 100644
--- a/dev-packages/e2e-tests/test-applications/tanstack-router/src/main.tsx
+++ b/dev-packages/e2e-tests/test-applications/tanstack-router/src/main.tsx
@@ -87,9 +87,25 @@ const redirectRoute = createRoute({
   },
 });
 
-const routeTree = rootRoute.addChildren([indexRoute, redirectRoute, postsRoute.addChildren([postIdRoute])]);
+// Dynamic enough to absorb basepath segments if they ever leak into route matching (see #23253).
+const catchAllRoute = createRoute({
+  getParentRoute: () => rootRoute,
+  path: '$a/$b/$c',
+  component: function CatchAll() {
+    return <div>Catch all</div>;
+  },
+});
+
+const routeTree = rootRoute.addChildren([
+  indexRoute,
+  redirectRoute,
+  catchAllRoute,
+  postsRoute.addChildren([postIdRoute]),
+]);
+
+declare const __APP_BASEPATH__: string;
 
-const router = createRouter({ routeTree });
+const router = createRouter({ routeTree, ...(__APP_BASEPATH__ ? { basepath: __APP_BASEPATH__ } : {}) });
 
 declare const __APP_DSN__: string;
 
diff --git a/dev-packages/e2e-tests/test-applications/tanstack-router/tests/basepath.test.ts b/dev-packages/e2e-tests/test-applications/tanstack-router/tests/basepath.test.ts
new file mode 100644
index 000000000000..bd3040c429a3
--- /dev/null
+++ b/dev-packages/e2e-tests/test-applications/tanstack-router/tests/basepath.test.ts
@@ -0,0 +1,59 @@
+import { expect, test } from '@playwright/test';
+import { waitForError, waitForTransaction } from '@sentry-internal/test-utils';
+
+// Only meaningful in the `tanstack-router (basepath)` variant, where the router is created with
+// `basepath: '/app'`. The rest of the suite runs in both variants.
+const BASE = process.env.E2E_TEST_BASEPATH || '';
+
+test.describe('router basepath', () => {
+  test.skip(!BASE, 'Only runs in the basepath variant');
+
+  // `window.location.pathname` carries the basepath, but the router never sees it. Matching the
+  // pageload against the raw browser path let the catch-all `/$a/$b/$c` route absorb `app` as a
+  // param instead of matching `/posts/$postId`.
+  test('does not leak the basepath into the matched route params', async ({ page }) => {
+    const transactionPromise = waitForTransaction('tanstack-router', async transactionEvent => {
+      return !!transactionEvent?.transaction && transactionEvent.contexts?.trace?.op === 'pageload';
+    });
+
+    await page.goto(`${BASE}/posts/456`);
+
+    const rootSpan = await transactionPromise;
+
+    // `onResolved` later merges the correct params in, but never clears the ones the bad initial
+    // match already set, so the stale `a`/`b`/`c` params survive on the span. Keys are passed as
+    // arrays because `toHaveProperty` would otherwise read the dots as a nested lookup.
+    const traceData = rootSpan.contexts?.trace?.data;
+    expect(traceData).not.toHaveProperty(['url.path.params.a']);
+    expect(traceData).not.toHaveProperty(['url.path.params.b']);
+    expect(traceData).not.toHaveProperty(['url.path.params.c']);
+    expect(traceData).toHaveProperty(['url.path.params.postId'], '456');
+    expect(traceData).toHaveProperty(['url.template'], '/posts/$postId');
+  });
+
+  // The first test only checks the span. The scope transaction is a separate value: it is set once
+  // when the pageload span starts, and the later `updateName` in `onResolved` does not rewrite it.
+  // So even when the sent transaction name is correct, errors captured after the pageload still
+  // carry the name from the initial match. This test checks that scope transaction.
+  test('attributes errors to the matched route for the whole page lifetime', async ({ page }) => {
+    const transactionPromise = waitForTransaction('tanstack-router', async transactionEvent => {
+      return !!transactionEvent?.transaction && transactionEvent.contexts?.trace?.op === 'pageload';
+    });
+    const errorPromise = waitForError('tanstack-router', async errorEvent => {
+      return errorEvent.exception?.values?.[0]?.value === 'Error thrown after pageload';
+    });
+
+    await page.goto(`${BASE}/posts/456`);
+    await transactionPromise;
+
+    await page.evaluate(() => {
+      setTimeout(() => {
+        throw new Error('Error thrown after pageload');
+      }, 0);
+    });
+
+    const errorEvent = await errorPromise;
+
+    expect(errorEvent.transaction).toBe('/posts/$postId');
+  });
+});
diff --git a/dev-packages/e2e-tests/test-applications/tanstack-router/tests/routing-instrumentation.test.ts b/dev-packages/e2e-tests/test-applications/tanstack-router/tests/routing-instrumentation.test.ts
index 06708292089e..13a1d859a828 100644
--- a/dev-packages/e2e-tests/test-applications/tanstack-router/tests/routing-instrumentation.test.ts
+++ b/dev-packages/e2e-tests/test-applications/tanstack-router/tests/routing-instrumentation.test.ts
@@ -1,12 +1,14 @@
 import { expect, test } from '@playwright/test';
 import { waitForTransaction } from '@sentry-internal/test-utils';
 
+const BASE = process.env.E2E_TEST_BASEPATH || '';
+
 test('sends a pageload transaction with a parameterized URL', async ({ page }) => {
   const transactionPromise = waitForTransaction('tanstack-router', async transactionEvent => {
     return !!transactionEvent?.transaction && transactionEvent.contexts?.trace?.op === 'pageload';
   });
 
-  await page.goto(`/posts/456`);
+  await page.goto(`${BASE}/posts/456`);
 
   const rootSpan = await transactionPromise;
 
@@ -43,7 +45,7 @@ test('sends pageload transaction with web vitals measurements', async ({ page })
     return !!transactionEvent?.transaction && transactionEvent.contexts?.trace?.op === 'pageload';
   });
 
-  await page.goto(`/`);
+  await page.goto(`${BASE}/`);
 
   const transaction = await transactionPromise;
 
@@ -94,7 +96,7 @@ test('sends a navigation transaction with a parameterized URL', async ({ page })
     return !!transactionEvent?.transaction && transactionEvent.contexts?.trace?.op === 'navigation';
   });
 
-  await page.goto(`/`);
+  await page.goto(`${BASE}/`);
   await pageloadTxnPromise;
 
   await page.waitForTimeout(5000);
@@ -138,7 +140,7 @@ test('sends a pageload transaction with resolved URL attrs after same-route redi
   });
 
   // `/posts/999` matches `/posts/$postId` initially, then `beforeLoad` redirects to `/posts/2`.
-  await page.goto(`/posts/999`);
+  await page.goto(`${BASE}/posts/999`);
 
   const pageloadTxn = await pageloadTxnPromise;
 
@@ -174,7 +176,7 @@ test('sends a pageload transaction named after the resolved route when a redirec
 
   // Visiting `/redirect` directly throws `redirect({ to: '/posts/$postId', params: { postId: '1' } })`
   // in `beforeLoad` during the initial pageload, so the pageload span must be renamed to the target route.
-  await page.goto(`/redirect`);
+  await page.goto(`${BASE}/redirect`);
 
   const pageloadTxn = await pageloadTxnPromise;
 
@@ -210,7 +212,7 @@ test('sends a navigation transaction when a redirect is thrown in beforeLoad', a
     return !!transactionEvent?.transaction && transactionEvent.contexts?.trace?.op === 'navigation';
   });
 
-  await page.goto(`/`);
+  await page.goto(`${BASE}/`);
   await pageloadTxnPromise;
 
   await page.locator('#redirect-link').click();
@@ -247,7 +249,7 @@ test('sends a navigation transaction for a normal navigation that happens after
     return !!transactionEvent?.transaction && transactionEvent.contexts?.trace?.op === 'pageload';
   });
 
-  await page.goto(`/`);
+  await page.goto(`${BASE}/`);
   await pageloadTxnPromise;
 
   // First trigger a redirect-driven navigation. Upstream (TanStack/router#3920) this leaves the
diff --git a/dev-packages/e2e-tests/test-applications/tanstack-router/vite.config.ts b/dev-packages/e2e-tests/test-applications/tanstack-router/vite.config.ts
index bd51f2f9679a..f863bf6d3085 100644
--- a/dev-packages/e2e-tests/test-applications/tanstack-router/vite.config.ts
+++ b/dev-packages/e2e-tests/test-applications/tanstack-router/vite.config.ts
@@ -1,11 +1,15 @@
 import react from '@vitejs/plugin-react-swc';
 import { defineConfig } from 'vite';
 
+const basepath = process.env.E2E_TEST_BASEPATH || '';
+
 // https://vitejs.dev/config/
 export default defineConfig({
+  base: basepath ? `${basepath}/` : '/',
   plugins: [react()],
   define: {
     __APP_DSN__: JSON.stringify(process.env.E2E_TEST_DSN),
+    __APP_BASEPATH__: JSON.stringify(basepath),
   },
   preview: {
     port: 3030,
diff --git a/packages/react/src/tanstackrouter.ts b/packages/react/src/tanstackrouter.ts
index ea6e9a3ea1f2..de6e8623db90 100644
--- a/packages/react/src/tanstackrouter.ts
+++ b/packages/react/src/tanstackrouter.ts
@@ -82,10 +82,13 @@ export function tanstackRouterBrowserTracingIntegration(
 
       const initialWindowLocation = WINDOW.location;
       if (instrumentPageLoad && initialWindowLocation) {
-        const routeMatch = resolveRouteMatch(
-          initialWindowLocation.pathname,
-          castRouterInstance.options.parseSearch(initialWindowLocation.search),
-        );
+        const initialRouterLocation = castRouterInstance.state?.location;
+        const routeMatch = initialRouterLocation
+          ? resolveRouteMatch(initialRouterLocation.pathname, initialRouterLocation.search)
+          : resolveRouteMatch(
+              initialWindowLocation.pathname,
+              castRouterInstance.options.parseSearch(initialWindowLocation.search),
+            );
 
         const pageloadSpan = startBrowserTracingPageLoadSpan(client, {
           name: routeMatch ? routeMatch.routeId : initialWindowLocation.pathname,
diff --git a/packages/react/src/vendor/tanstackrouter-types.ts b/packages/react/src/vendor/tanstackrouter-types.ts
index 3936c429cd5d..c8b296707d10 100644
--- a/packages/react/src/vendor/tanstackrouter-types.ts
+++ b/packages/react/src/vendor/tanstackrouter-types.ts
@@ -45,6 +45,7 @@ interface VendoredTanstackRouterHistory {
 interface VendoredTanstackRouterState {
   matches: Array<VendoredTanstackRouterRouteMatch>;
   pendingMatches?: Array<VendoredTanstackRouterRouteMatch>;
+  location?: VendoredTanstackRouterLocation;
 }
 
 export interface VendoredTanstackRouterRouteMatch {
diff --git a/packages/react/test/tanstackrouter.test.ts b/packages/react/test/tanstackrouter.test.ts
index c45bd83753c0..c75e9156166c 100644
--- a/packages/react/test/tanstackrouter.test.ts
+++ b/packages/react/test/tanstackrouter.test.ts
@@ -58,6 +58,7 @@ describe('tanstackRouterBrowserTracingIntegration', () => {
   beforeEach(() => {
     vi.clearAllMocks();
     startBrowserTracingPageLoadSpanSpy.mockReturnValue(mockPageloadSpan as any);
+    (SentryBrowser.WINDOW as any).location = { pathname: '/posts/999', search: '' };
 
     vi.stubGlobal('window', {
       location: {
@@ -91,6 +92,39 @@ describe('tanstackRouterBrowserTracingIntegration', () => {
     });
   });
 
+  describe('pageload route matching', () => {
+    // `window.location.pathname` carries the router basepath, but whether `matchRoutes` wants it is
+    // version-dependent (newer routers strip it in `parseLocation`, older ones inside `matchRoutes`).
+    // `state.location` is always in the form the router itself expects, so we match against that.
+    it('matches against the router location, not window.location', () => {
+      (SentryBrowser.WINDOW as any).location = { pathname: '/app/posts/999', search: '?q=1' };
+
+      const integration = tanstackRouterBrowserTracingIntegration(
+        { ...mockRouter, state: { location: { pathname: '/posts/999', search: { q: 1 } } } },
+        { instrumentPageLoad: true, instrumentNavigation: false },
+      );
+
+      integration.afterAllSetup!(mockClient as any);
+
+      expect(mockRouter.matchRoutes).toHaveBeenCalledWith('/posts/999', { q: 1 }, expect.any(Object));
+      expect(mockRouter.options.parseSearch).not.toHaveBeenCalled();
+    });
+
+    it('falls back to window.location when the router exposes no location', () => {
+      (SentryBrowser.WINDOW as any).location = { pathname: '/posts/999', search: '?q=1' };
+
+      const integration = tanstackRouterBrowserTracingIntegration(mockRouter, {
+        instrumentPageLoad: true,
+        instrumentNavigation: false,
+      });
+
+      integration.afterAllSetup!(mockClient as any);
+
+      expect(mockRouter.options.parseSearch).toHaveBeenCalledWith('?q=1');
+      expect(mockRouter.matchRoutes).toHaveBeenCalledWith('/posts/999', {}, expect.any(Object));
+    });
+  });
+
   it('updates pageload span URL attributes on redirect to the same route template', () => {
     const integration = tanstackRouterBrowserTracingIntegration(mockRouter, {
       instrumentPageLoad: true,