diff --git a/packages/react-native/Libraries/ReactNative/AppContainer-dev.js b/packages/react-native/Libraries/ReactNative/AppContainer-dev.js
index 7eee9c75f8e7..db3e0868daa7 100644
--- a/packages/react-native/Libraries/ReactNative/AppContainer-dev.js
+++ b/packages/react-native/Libraries/ReactNative/AppContainer-dev.js
@@ -100,7 +100,6 @@ const ReactDevToolsOverlayDeferred = ({
 
 const AppContainer = ({
   children,
-  fabric,
   initialProps,
   internal_excludeInspector = false,
   internal_excludeLogBox = false,
@@ -165,7 +164,7 @@ const AppContainer = ({
 
   if (WrapperComponent != null) {
     innerView = (
-      <WrapperComponent initialProps={initialProps} fabric={fabric === true}>
+      <WrapperComponent initialProps={initialProps}>
         {innerView}
       </WrapperComponent>
     );
diff --git a/packages/react-native/Libraries/ReactNative/AppContainer-prod.js b/packages/react-native/Libraries/ReactNative/AppContainer-prod.js
index 5f8de6fe7839..bd3ea7c4ab9b 100644
--- a/packages/react-native/Libraries/ReactNative/AppContainer-prod.js
+++ b/packages/react-native/Libraries/ReactNative/AppContainer-prod.js
@@ -17,7 +17,6 @@ import * as React from 'react';
 
 const AppContainer = ({
   children,
-  fabric,
   initialProps,
   rootTag,
   WrapperComponent,
@@ -27,7 +26,7 @@ const AppContainer = ({
 
   if (WrapperComponent != null) {
     innerView = (
-      <WrapperComponent initialProps={initialProps} fabric={fabric === true}>
+      <WrapperComponent initialProps={initialProps}>
         {innerView}
       </WrapperComponent>
     );
diff --git a/packages/react-native/Libraries/ReactNative/AppContainer.js b/packages/react-native/Libraries/ReactNative/AppContainer.js
index 58bc7319e44c..a9e6fff6541e 100644
--- a/packages/react-native/Libraries/ReactNative/AppContainer.js
+++ b/packages/react-native/Libraries/ReactNative/AppContainer.js
@@ -15,7 +15,6 @@ import * as React from 'react';
 
 export type Props = Readonly<{
   children?: React.Node,
-  fabric?: boolean,
   rootTag: number | RootTag,
   initialProps?: {...},
   WrapperComponent?: ?React.ComponentType<any>,
diff --git a/packages/react-native/Libraries/ReactNative/AppRegistry.flow.js b/packages/react-native/Libraries/ReactNative/AppRegistry.flow.js
index ce51f1380c9f..948ef3b5c859 100644
--- a/packages/react-native/Libraries/ReactNative/AppRegistry.flow.js
+++ b/packages/react-native/Libraries/ReactNative/AppRegistry.flow.js
@@ -31,7 +31,6 @@ export type AppConfig = {
 export type AppParameters = {
   initialProps: Readonly<{[string]: unknown, ...}>,
   rootTag: RootTag,
-  fabric?: boolean,
 };
 export type Runnable = (
   appParameters: AppParameters,
diff --git a/packages/react-native/Libraries/ReactNative/AppRegistryImpl.js b/packages/react-native/Libraries/ReactNative/AppRegistryImpl.js
index c37640b1b5dc..e6bd106713d6 100644
--- a/packages/react-native/Libraries/ReactNative/AppRegistryImpl.js
+++ b/packages/react-native/Libraries/ReactNative/AppRegistryImpl.js
@@ -94,7 +94,7 @@ export function registerComponent(
       appParameters.rootTag,
       wrapperComponentProvider && wrapperComponentProvider(appParameters),
       rootViewStyleProvider && rootViewStyleProvider(appParameters),
-      appParameters.fabric,
+      true, // fabric - deprecated, always true
       scopedPerformanceLogger,
       appKey === 'LogBox', // is logbox
       appKey,
diff --git a/packages/react-native/Libraries/ReactNative/renderApplication.js b/packages/react-native/Libraries/ReactNative/renderApplication.js
index 875e3fef4be0..1dd4e52e1cc1 100644
--- a/packages/react-native/Libraries/ReactNative/renderApplication.js
+++ b/packages/react-native/Libraries/ReactNative/renderApplication.js
@@ -13,7 +13,6 @@ import type {IPerformanceLogger} from '../Utilities/createPerformanceLogger';
 
 import GlobalPerformanceLogger from '../Utilities/GlobalPerformanceLogger';
 import PerformanceLoggerContext from '../Utilities/PerformanceLoggerContext';
-import warnOnce from '../Utilities/warnOnce';
 import AppContainer from './AppContainer';
 import DisplayMode, {type DisplayModeType} from './DisplayMode';
 import getCachedComponentWithDebugName from './getCachedComponentWithDebugName';
@@ -37,7 +36,9 @@ export default function renderApplication<Props extends Object>(
   rootTag: any,
   WrapperComponent?: ?React.ComponentType<any>,
   rootViewStyle?: ?ViewStyleProp,
-  fabric?: boolean,
+  // Keep this parameter for backwards compatibility only. It is always treated as
+  // true internally.
+  fabric?: true | void,
   scopedPerformanceLogger?: IPerformanceLogger,
   isLogBox?: boolean,
   debugName?: string,
@@ -52,7 +53,6 @@ export default function renderApplication<Props extends Object>(
     <PerformanceLoggerContext.Provider value={performanceLogger}>
       <AppContainer
         rootTag={rootTag}
-        fabric={fabric}
         WrapperComponent={WrapperComponent}
         rootViewStyle={rootViewStyle}
         initialProps={initialProps ?? Object.freeze({})}
@@ -87,15 +87,9 @@ export default function renderApplication<Props extends Object>(
     );
   }
 
-  // We want to have concurrentRoot always enabled when you're on Fabric.
-  const useConcurrentRoot = Boolean(fabric);
-
   performanceLogger.startTimespan('renderApplication_React_render');
-  performanceLogger.setExtra(
-    'usedReactConcurrentRoot',
-    useConcurrentRoot ? '1' : '0',
-  );
-  performanceLogger.setExtra('usedReactFabric', fabric ? '1' : '0');
+  performanceLogger.setExtra('usedReactConcurrentRoot', '1');
+  performanceLogger.setExtra('usedReactFabric', '1');
   performanceLogger.setExtra(
     'usedReactProfiler',
     Renderer.isProfilingRenderer(),
@@ -103,16 +97,8 @@ export default function renderApplication<Props extends Object>(
   Renderer.renderElement({
     element: renderable,
     rootTag,
-    useFabric: Boolean(fabric),
-    useConcurrentRoot,
+    useFabric: true,
+    useConcurrentRoot: true,
   });
-
-  const newArchitecture = !!fabric;
-  if (!newArchitecture) {
-    warnOnce(
-      '[OSS][OldArchDeprecatedWarning]',
-      'The app is running using the Legacy Architecture. The Legacy Architecture is deprecated and will be removed in a future version of React Native. Please consider migrating to the New Architecture. For more information, please see https://reactnative.dev/blog/2024/10/23/the-new-architecture-is-here',
-    );
-  }
   performanceLogger.stopTimespan('renderApplication_React_render');
 }
diff --git a/packages/react-native/ReactNativeApi.d.ts b/packages/react-native/ReactNativeApi.d.ts
index fdd39000be66..9d852f4d2f62 100644
--- a/packages/react-native/ReactNativeApi.d.ts
+++ b/packages/react-native/ReactNativeApi.d.ts
@@ -4,7 +4,7 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  *
- * @generated SignedSource<<c018df63c1391fd6906f66f9bb80bb27>>
+ * @generated SignedSource<<ad3edd7df03c5d97bc1bbfc138e3b283>>
  *
  * This file was generated by scripts/js-api/build-types/index.js.
  */
@@ -1638,7 +1638,6 @@ declare type AppearancePreferences = {
   colorScheme?: ColorSchemeName
 }
 declare type AppParameters = {
-  fabric?: boolean
   initialProps: {
     readonly [$$Key$$: string]: unknown
   }
@@ -6013,8 +6012,8 @@ export {
   AlertType, // 5ab91217
   AndroidKeyboardEvent, // e03becc8
   Animated, // a77a3944
-  AppConfig, // ebddad4b
-  AppRegistry, // f7a253e4
+  AppConfig, // ce4209a7
+  AppRegistry, // 5edf0524
   AppState, // 12012be5
   AppStateEvent, // 80f034c3
   AppStateStatus, // 447e5ef2
@@ -6183,15 +6182,15 @@ export {
   RefreshControlProps, // e747ed5d
   RefreshControlPropsAndroid, // 99f64c97
   RefreshControlPropsIOS, // 72a36381
-  Registry, // e1ed403e
+  Registry, // 6c39216d
   ResponderSyntheticEvent, // fb10247c
   ReturnKeyTypeOptions, // afd47ba3
   Role, // af7b889d
   RootTag, // 3cd10504
   RootTagContext, // 38bfc8f6
   RootViewStyleProvider, // d4818465
-  Runnable, // 2cb32c54
-  Runnables, // d3749ae1
+  Runnable, // 594dd93a
+  Runnables, // 4367c557
   SafeAreaView, // 9589fa67
   ScaledSize, // 07e417c7
   ScrollEvent, // 5d529218