diff --git a/packages/react-native/src/private/webapis/performance/__tests__/Performance-itest.js b/packages/react-native/src/private/webapis/performance/__tests__/Performance-itest.js
index 1572338f3c04..8e2ea9654f56 100644
--- a/packages/react-native/src/private/webapis/performance/__tests__/Performance-itest.js
+++ b/packages/react-native/src/private/webapis/performance/__tests__/Performance-itest.js
@@ -10,6 +10,8 @@
 
 import '@react-native/fantom/src/setUpDefaultReactNativeEnvironment';
 
+import * as Fantom from '@react-native/fantom';
+
 describe('Performance', () => {
   it('does NOT allow creating instances of Performance directly', () => {
     expect(() => {
@@ -40,7 +42,24 @@ describe('Performance', () => {
   });
 
   describe('timeOrigin', () => {
-    it('allows moving timestamps to Unix epoch', () => {
+    it('is a positive number that does not change between calls', () => {
+      const first = performance.timeOrigin;
+      const second = performance.timeOrigin;
+
+      expect(typeof first).toBe('number');
+      expect(first).toBeGreaterThan(0);
+      expect(second).toBe(first);
+    });
+
+    // On macOS, `performance.now()` is backed by a monotonic clock that does
+    // NOT include time spent while the system is asleep. This causes the
+    // monotonic time to drift relative to wall time (`Date.now()`) the longer
+    // the machine has been running, so we can't reliably compare
+    // `performance.now() + performance.timeOrigin` against `Date.now()` like
+    // we can on other platforms.
+    const itIfNotMacOS = Fantom.getHostPlatform() === 'macos' ? it.skip : it;
+
+    itIfNotMacOS('allows moving timestamps to Unix epoch', () => {
       // We need to truncate timestamps because `Date.now()` only provides
       // integer millisecond precision.
       const adjustedMonotonicTime = Math.trunc(
diff --git a/private/react-native-fantom/runner/entrypoint-template.js b/private/react-native-fantom/runner/entrypoint-template.js
index 2010fb54a46b..0c3c3becb04d 100644
--- a/private/react-native-fantom/runner/entrypoint-template.js
+++ b/private/react-native-fantom/runner/entrypoint-template.js
@@ -9,12 +9,32 @@
  */
 
 import type {SnapshotConfig} from '../runtime/snapshotContext';
-import type {FantomRuntimeConstants} from '../src/Constants';
+import type {FantomRuntimeConstants, HostPlatform} from '../src/Constants';
 import type {FantomTestConfig} from './getFantomTestConfigs';
 
 import * as EnvironmentOptions from './EnvironmentOptions';
 import formatFantomConfig from './formatFantomConfig';
 
+function getHostPlatform(): HostPlatform {
+  match (process.platform) {
+    'darwin' => {
+      return 'macos';
+    }
+    'win32' => {
+      return 'windows';
+    }
+    'linux' => {
+      return 'linux';
+    }
+    'android' => {
+      return 'android';
+    }
+    _ => {
+      throw new Error(`Unsupported platform: ${process.platform}`);
+    }
+  }
+}
+
 module.exports = function entrypointTemplate({
   testPath,
   setupModulePath,
@@ -42,6 +62,7 @@ module.exports = function entrypointTemplate({
     jsTraceOutputPath,
     jsHeapSnapshotOutputPathTemplate,
     jsHeapSnapshotOutputPathTemplateToken,
+    hostPlatform: getHostPlatform(),
   };
 
   return `/**
diff --git a/private/react-native-fantom/src/Constants.js b/private/react-native-fantom/src/Constants.js
index ffd8f3504df9..8ef0bb2af88b 100644
--- a/private/react-native-fantom/src/Constants.js
+++ b/private/react-native-fantom/src/Constants.js
@@ -8,6 +8,8 @@
  * @format
  */
 
+export type HostPlatform = 'android' | 'windows' | 'macos' | 'linux';
+
 export type FantomRuntimeConstants = Readonly<{
   isOSS: boolean,
   isRunningFromCI: boolean,
@@ -16,6 +18,7 @@ export type FantomRuntimeConstants = Readonly<{
   jsHeapSnapshotOutputPathTemplate: string,
   jsHeapSnapshotOutputPathTemplateToken: string,
   jsTraceOutputPath: ?string,
+  hostPlatform: HostPlatform,
 }>;
 
 let constants: FantomRuntimeConstants = {
@@ -26,6 +29,7 @@ let constants: FantomRuntimeConstants = {
   jsHeapSnapshotOutputPathTemplate: '',
   jsHeapSnapshotOutputPathTemplateToken: '',
   jsTraceOutputPath: null,
+  hostPlatform: 'linux',
 };
 
 export function getConstants(): FantomRuntimeConstants {
diff --git a/private/react-native-fantom/src/index.js b/private/react-native-fantom/src/index.js
index 73e40ba9d661..e290f00c492b 100644
--- a/private/react-native-fantom/src/index.js
+++ b/private/react-native-fantom/src/index.js
@@ -8,6 +8,7 @@
  * @format
  */
 
+import type {HostPlatform} from './Constants';
 import type {
   FantomRenderedOutput,
   RenderOutputConfig,
@@ -46,6 +47,16 @@ export type RootConfig = {
 
 export {getConstants} from './Constants';
 
+/**
+ * Returns the host OS where the Fantom test runner is running (e.g.
+ * 'linux', 'macos', 'windows'). This is different from React Native's
+ * `Platform.OS`, which always reflects the React Native target platform
+ * being tested.
+ */
+export function getHostPlatform(): HostPlatform {
+  return getConstants().hostPlatform;
+}
+
 // Defaults use iPhone 14 values (very common device).
 const DEFAULT_VIEWPORT_WIDTH = 390;
 const DEFAULT_VIEWPORT_HEIGHT = 844;