diff --git a/CHANGELOG.md b/CHANGELOG.md
index ef73608dcb..c53be5232d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -10,6 +10,8 @@ This is the log of notable changes to EAS CLI and related packages.
 
 ### 🐛 Bug fixes
 
+- [build-tools] Install ffmpeg for Appium simulator sessions. ([#4577](https://github.com/expo/eas-cli/pull/4577) by [@gwdp](https://github.com/gwdp))
+
 ### 🧹 Chores
 
 ## [24.12.1](https://github.com/expo/eas-cli/releases/tag/v24.12.1) - 2026-10-08
diff --git a/packages/build-tools/src/steps/functions/__tests__/deviceSessionHost-orchestration.test.ts b/packages/build-tools/src/steps/functions/__tests__/deviceSessionHost-orchestration.test.ts
index 56fdefddf8..853e919e09 100644
--- a/packages/build-tools/src/steps/functions/__tests__/deviceSessionHost-orchestration.test.ts
+++ b/packages/build-tools/src/steps/functions/__tests__/deviceSessionHost-orchestration.test.ts
@@ -260,3 +260,76 @@ describe.each([
     );
   });
 });
+
+describe('Appium ffmpeg setup', () => {
+  async function runAppiumAsync(): Promise<void> {
+    const fn = createStartAppiumRemoteSessionBuildFunction({} as CustomBuildContext);
+    await fn.fn!(
+      {
+        logger,
+        global: { runtimePlatform: BuildRuntimePlatform.LINUX },
+      } as unknown as BuildStepContext,
+      {
+        inputs: new Proxy({} as Record<string, { value: unknown }>, {
+          get: (target, id: string) => target[id] ?? { value: undefined },
+        }),
+        outputs: {},
+        env: {
+          DEVICE_RUN_SESSION_ID: 'session-id',
+          EAS_SIMULATOR_NGROK_TUNNEL_DOMAIN: 'example.test',
+          NGROK_AUTHTOKEN: 'token',
+        },
+      } as never
+    );
+  }
+
+  function holdFfmpegInstall({ appiumError }: { appiumError?: Error } = {}) {
+    let finish!: (outcome: 'installed' | 'failed') => void;
+    const install = new Promise((resolve, reject) => {
+      finish = outcome =>
+        outcome === 'installed' ? resolve({}) : reject(new Error('apt-get failed'));
+    });
+    jest.mocked(spawn).mockImplementation(((command: string, args: string[]) => {
+      if (command === 'ffmpeg') {
+        return Promise.reject(new Error('ffmpeg missing'));
+      }
+      if (args.includes('install') && args.includes('ffmpeg')) {
+        return install;
+      }
+      return command === 'npm' && appiumError
+        ? Promise.reject(appiumError)
+        : Promise.resolve({ stdout: '{}' });
+    }) as never);
+    return finish;
+  }
+
+  it.each(['installed', 'failed'] as const)(
+    'starts Appium after the ffmpeg install settles (%s)',
+    async outcome => {
+      const finishFfmpegInstall = holdFfmpegInstall();
+
+      const run = runAppiumAsync();
+      await new Promise(resolve => setImmediate(resolve));
+      expect(spawnDetached).not.toHaveBeenCalled();
+
+      finishFfmpegInstall(outcome);
+      await run;
+      expect(spawnDetached).toHaveBeenCalledTimes(1);
+    }
+  );
+
+  it('waits for the ffmpeg install before failing on an Appium install error', async () => {
+    const appiumError = new Error('npm failed');
+    const finishFfmpegInstall = holdFfmpegInstall({ appiumError });
+
+    let settled = false;
+    const run = runAppiumAsync().finally(() => {
+      settled = true;
+    });
+    await new Promise(resolve => setImmediate(resolve));
+    expect(settled).toBe(false);
+
+    finishFfmpegInstall('installed');
+    await expect(run).rejects.toBe(appiumError);
+  });
+});
diff --git a/packages/build-tools/src/steps/functions/startAppiumRemoteSession.ts b/packages/build-tools/src/steps/functions/startAppiumRemoteSession.ts
index ccb79e4382..ad780ead68 100644
--- a/packages/build-tools/src/steps/functions/startAppiumRemoteSession.ts
+++ b/packages/build-tools/src/steps/functions/startAppiumRemoteSession.ts
@@ -1,5 +1,6 @@
 import { SystemError } from '@expo/eas-build-job';
 import { type bunyan } from '@expo/logger';
+import { asyncResult } from '@expo/results';
 import {
   BuildFunction,
   BuildRuntimePlatform,
@@ -37,6 +38,7 @@ import { startAppiumEventCollectionAsync } from '../utils/appiumEvents';
 import {
   createServeSimLaunchInputProviders,
   describeServeSimLaunch,
+  ensureFfmpegInstalledOnceAsync,
   finishRemoteSessionAsync,
   getDeviceRunSessionIdOrThrow,
   getNgrokAuthtokenOrThrow,
@@ -108,12 +110,17 @@ export function createStartAppiumRemoteSessionBuildFunction(
         `Starting Appium remote session (version: ${versionSpec}, runtime: ${runtimePlatform}).`
       );
       const device = await resolveAppiumDeviceAsync({ runtimePlatform, env, logger });
-      const { appiumHome, appiumBinPath, appiumEnv } = await installAppiumAsync({
-        versionSpec,
-        driverName: device.driverName,
-        env,
-        logger,
-      });
+      // Appium's startRecordingScreen runs ffmpeg on this host: XCUITest encodes the
+      // simulator stream with it, and UiAutomator2 merges long recordings with it. The
+      // macOS session image does not ship ffmpeg, so install it concurrently with Appium.
+      // Both installs settle before the step continues or fails, so no install outlives it.
+      const [appiumInstall] = await Promise.all([
+        asyncResult(
+          installAppiumAsync({ versionSpec, driverName: device.driverName, env, logger })
+        ),
+        ensureFfmpegInstalledOnceAsync({ runtimePlatform, env, logger }),
+      ]);
+      const { appiumHome, appiumBinPath, appiumEnv } = appiumInstall.enforceValue();
 
       const appiumProcess = spawnDetached({
         command: appiumBinPath,