diff --git a/packages/replay-internal/src/replay.ts b/packages/replay-internal/src/replay.ts
index 6588a9e4d79d..16f7a6e7d884 100644
--- a/packages/replay-internal/src/replay.ts
+++ b/packages/replay-internal/src/replay.ts
@@ -28,6 +28,7 @@ import { clearSession } from './session/clearSession';
 import { loadOrCreateSession } from './session/loadOrCreateSession';
 import { saveSession } from './session/saveSession';
 import { shouldRefreshSession } from './session/shouldRefreshSession';
+
 import type {
   AddEventResult,
   AddUpdateCallback,
@@ -295,6 +296,12 @@ export class ReplayContainer implements ReplayContainerInterface {
 
     logInfoNextTick('[Replay] Starting replay in session mode', this._options._experiments.traceInternals);
 
+    // Required as user activity is initially set in
+    // constructor, so if `start()` is called after
+    // session idle expiration, a replay will not be
+    // created due to an idle timeout.
+    this._updateUserActivity();
+
     const session = loadOrCreateSession(
       {
         maxReplayDuration: this._options.maxReplayDuration,
diff --git a/packages/replay-internal/test/integration/start.test.ts b/packages/replay-internal/test/integration/start.test.ts
new file mode 100644
index 000000000000..dff5df38b53d
--- /dev/null
+++ b/packages/replay-internal/test/integration/start.test.ts
@@ -0,0 +1,52 @@
+import { vi } from 'vitest';
+
+import { getClient } from '@sentry/core';
+import type { Transport } from '@sentry/types';
+
+import { DEFAULT_FLUSH_MIN_DELAY, SESSION_IDLE_EXPIRE_DURATION } from '../../src/constants';
+import type { Replay } from '../../src/integration';
+import type { ReplayContainer } from '../../src/replay';
+import { BASE_TIMESTAMP } from '../index';
+import { resetSdkMock } from '../mocks/resetSdkMock';
+import { useFakeTimers } from '../utils/use-fake-timers';
+
+useFakeTimers();
+
+describe('Integration | start', () => {
+  let replay: ReplayContainer;
+  let integration: Replay;
+
+  beforeEach(async () => {
+    ({ replay, integration } = await resetSdkMock({
+      replayOptions: {
+        stickySession: false,
+      },
+      sentryOptions: {
+        replaysSessionSampleRate: 0.0,
+      },
+    }));
+
+    const mockTransport = getClient()?.getTransport()?.send as vi.MockedFunction<Transport['send']>;
+    mockTransport?.mockClear();
+    await vi.runAllTimersAsync();
+  });
+
+  afterEach(async () => {
+    integration.stop();
+
+    await vi.runAllTimersAsync();
+    vi.setSystemTime(new Date(BASE_TIMESTAMP));
+  });
+
+  it('sends replay when calling `start()` after [SESSION_IDLE_EXPIRE_DURATION]ms', async () => {
+    await vi.advanceTimersByTimeAsync(SESSION_IDLE_EXPIRE_DURATION + 1);
+
+    integration.start();
+
+    await vi.advanceTimersByTimeAsync(DEFAULT_FLUSH_MIN_DELAY);
+
+    expect(replay).toHaveLastSentReplay({
+      recordingPayloadHeader: { segment_id: 0 },
+    });
+  });
+});