diff --git a/jest.config.js b/jest.config.js
index baee6fe96ed8..240abcbfeae2 100644
--- a/jest.config.js
+++ b/jest.config.js
@@ -43,7 +43,7 @@ module.exports = {
     '<rootDir>/packages/react-native/Libraries/Renderer',
     '<rootDir>/packages/react-native/sdks/hermes/',
     ...PODS_LOCATIONS,
-  ] /*:: as $ReadOnlyArray<string> */,
+  ] /*:: as ReadonlyArray<string> */,
   transformIgnorePatterns: ['node_modules/(?!@react-native/)'],
   haste: {
     defaultPlatform: 'ios',
@@ -52,12 +52,12 @@ module.exports = {
   moduleFileExtensions: [
     'fb.js',
     ...defaults.moduleFileExtensions,
-  ] /*:: as $ReadOnlyArray<string> */,
+  ] /*:: as ReadonlyArray<string> */,
   modulePathIgnorePatterns: [
     'scripts/.*/__fixtures__/',
     '<rootDir>/packages/react-native/sdks/hermes/',
     ...PODS_LOCATIONS,
-  ] /*:: as $ReadOnlyArray<string> */,
+  ] /*:: as ReadonlyArray<string> */,
   unmockedModulePathPatterns: [
     'node_modules/react/',
     'packages/react-native/Libraries/Renderer',
diff --git a/packages/dev-middleware/src/__tests__/InspectorProxyCdpTransport-test.js b/packages/dev-middleware/src/__tests__/InspectorProxyCdpTransport-test.js
index 6a5e6a23a158..7e33c2decb59 100644
--- a/packages/dev-middleware/src/__tests__/InspectorProxyCdpTransport-test.js
+++ b/packages/dev-middleware/src/__tests__/InspectorProxyCdpTransport-test.js
@@ -522,9 +522,9 @@ describe.each(['HTTP', 'HTTPS'])(
 
           // Collect all connect events
           const connectEvents: Array<
-            $ReadOnly<{
+            Readonly<{
               event: string,
-              payload: $ReadOnly<{pageId: string, sessionId?: string}>,
+              payload: Readonly<{pageId: string, sessionId?: string}>,
             }>,
           > = [];
           device.connect.mockImplementation(message => {
diff --git a/packages/dev-middleware/src/types/ReadonlyURL.js b/packages/dev-middleware/src/types/ReadonlyURL.js
index 9ff197843673..b8c46a4c9c5f 100644
--- a/packages/dev-middleware/src/types/ReadonlyURL.js
+++ b/packages/dev-middleware/src/types/ReadonlyURL.js
@@ -25,7 +25,7 @@ export interface ReadonlyURLSearchParams {
       value: string,
       name: string,
       params: URLSearchParams,
-    ) => mixed,
+    ) => unknown,
     thisArg: This,
   ): void;
   toString(): string;
diff --git a/packages/react-native-babel-preset/src/__tests__/__fixtures__/kitchen-sink-input.js b/packages/react-native-babel-preset/src/__tests__/__fixtures__/kitchen-sink-input.js
index d6cf667105c3..c87fd6c1ae7d 100644
--- a/packages/react-native-babel-preset/src/__tests__/__fixtures__/kitchen-sink-input.js
+++ b/packages/react-native-babel-preset/src/__tests__/__fixtures__/kitchen-sink-input.js
@@ -59,7 +59,7 @@ async function* asyncNumberGenerator(
 }
 
 // Async/await patterns
-async function fetchData(url: string): Promise<{data: mixed}> {
+async function fetchData(url: string): Promise<{data: unknown}> {
   const response = await fetch(url);
   const data = await response.json();
   return {data};
@@ -159,7 +159,7 @@ function parseDate(
 }
 
 // Try-catch with optional binding
-function safeJsonParse(input: string): mixed {
+function safeJsonParse(input: string): unknown {
   try {
     return JSON.parse(input);
   } catch {
@@ -182,7 +182,7 @@ const MyClass = class {
 };
 
 // Dynamic import (syntax only)
-async function loadModule(): Promise<mixed> {
+async function loadModule(): Promise<unknown> {
   // $FlowExpectedError[cannot-resolve-module] - Testing dynamic import syntax
   const module = await import('./some-module');
   return module.default;
diff --git a/packages/react-native-babel-preset/src/__tests__/transform-snapshot-test.js b/packages/react-native-babel-preset/src/__tests__/transform-snapshot-test.js
index 435f27d40b24..32b7deace2c3 100644
--- a/packages/react-native-babel-preset/src/__tests__/transform-snapshot-test.js
+++ b/packages/react-native-babel-preset/src/__tests__/transform-snapshot-test.js
@@ -133,7 +133,7 @@ const testConfigs = [
 
 function transformCode(
   code: string,
-  options: {[string]: mixed},
+  options: {[string]: unknown},
 ): string | null {
   const result = babel.transformSync(code, {
     babelrc: false,
@@ -156,7 +156,7 @@ function ensureDirectoryExists(dir: string): void {
   }
 }
 
-function makeHeader(description: string, options: {[string]: mixed}): string {
+function makeHeader(description: string, options: {[string]: unknown}): string {
   return `/**
  * Copyright (c) Meta Platforms, Inc. and affiliates.
  *
diff --git a/packages/react-native-codegen/src/parsers/consistency/compareSnaps.js b/packages/react-native-codegen/src/parsers/consistency/compareSnaps.js
index 052eb2041e91..448065d0ed14 100644
--- a/packages/react-native-codegen/src/parsers/consistency/compareSnaps.js
+++ b/packages/react-native-codegen/src/parsers/consistency/compareSnaps.js
@@ -11,13 +11,13 @@
 'use strict';
 
 function compareSnaps(
-  flowFixtures /*: $ReadOnly<{[string]: string}> */,
-  flowSnaps /*: $ReadOnly<{[string]: string}> */,
-  flowExtraCases /*: $ReadOnlyArray<string> */,
-  tsFixtures /*: $ReadOnly<{[string]: string}> */,
-  tsSnaps /*: $ReadOnly<{[string]: string}> */,
-  tsExtraCases /*: $ReadOnlyArray<string> */,
-  ignoredCases /*: $ReadOnlyArray<string> */,
+  flowFixtures /*: Readonly<{[string]: string}> */,
+  flowSnaps /*: Readonly<{[string]: string}> */,
+  flowExtraCases /*: ReadonlyArray<string> */,
+  tsFixtures /*: Readonly<{[string]: string}> */,
+  tsSnaps /*: Readonly<{[string]: string}> */,
+  tsExtraCases /*: ReadonlyArray<string> */,
+  ignoredCases /*: ReadonlyArray<string> */,
 ) {
   const flowCases = Object.keys(flowFixtures).sort();
   const tsCases = Object.keys(tsFixtures).sort();
@@ -68,8 +68,8 @@ function compareSnaps(
 }
 
 function compareTsArraySnaps(
-  tsSnaps /*: $ReadOnly<{[string]: string}> */,
-  tsExtraCases /*: $ReadOnlyArray<string> */,
+  tsSnaps /*: Readonly<{[string]: string}> */,
+  tsExtraCases /*: ReadonlyArray<string> */,
 ) {
   for (const array2Case of tsExtraCases.filter(
     name => name.indexOf('ARRAY2') !== -1,
diff --git a/packages/react-native-compatibility-check/src/ErrorFormatting.js b/packages/react-native-compatibility-check/src/ErrorFormatting.js
index 64d0d36a6866..8f7fb95770d3 100644
--- a/packages/react-native-compatibility-check/src/ErrorFormatting.js
+++ b/packages/react-native-compatibility-check/src/ErrorFormatting.js
@@ -199,7 +199,7 @@ function formatTypeAnnotation(annotation: CompleteTypeAnnotation): string {
       let validUnionType;
       try {
         validUnionType = parseValidUnionType(annotation);
-      } catch (_e: mixed) {
+      } catch (_e: unknown) {
         // parseValidUnionType throws for unsupported union types
         return 'Union<mixed>';
       }
diff --git a/packages/react-native/Libraries/Components/AccessibilityInfo/__tests__/AccessibilityInfo-test.js b/packages/react-native/Libraries/Components/AccessibilityInfo/__tests__/AccessibilityInfo-test.js
index a9ca14cbf0b5..82460fe77383 100644
--- a/packages/react-native/Libraries/Components/AccessibilityInfo/__tests__/AccessibilityInfo-test.js
+++ b/packages/react-native/Libraries/Components/AccessibilityInfo/__tests__/AccessibilityInfo-test.js
@@ -105,7 +105,7 @@ describe('AccessibilityInfo', () => {
         mockNativeAccessibilityManagerDefault.getCurrentPrefersCrossFadeTransitionsState =
           null;
 
-        const result: mixed =
+        const result: unknown =
           await AccessibilityInfo.prefersCrossFadeTransitions().catch(e => e);
 
         invariant(
@@ -151,7 +151,7 @@ describe('AccessibilityInfo', () => {
         mockNativeAccessibilityManagerDefault.getCurrentDarkerSystemColorsState =
           null;
 
-        const result: mixed =
+        const result: unknown =
           await AccessibilityInfo.isDarkerSystemColorsEnabled().catch(e => e);
 
         invariant(
@@ -184,7 +184,7 @@ describe('AccessibilityInfo', () => {
 
         mockNativeAccessibilityInfo.isHighTextContrastEnabled = null;
 
-        const result: mixed =
+        const result: unknown =
           await AccessibilityInfo.isHighTextContrastEnabled().catch(e => e);
 
         invariant(
diff --git a/packages/react-native/Libraries/Core/Devtools/__tests__/loadBundleFromServer-test.js b/packages/react-native/Libraries/Core/Devtools/__tests__/loadBundleFromServer-test.js
index 92827be1d974..7657099fd2e1 100644
--- a/packages/react-native/Libraries/Core/Devtools/__tests__/loadBundleFromServer-test.js
+++ b/packages/react-native/Libraries/Core/Devtools/__tests__/loadBundleFromServer-test.js
@@ -95,7 +95,7 @@ test('loadBundleFromServer will throw for JSON responses', async () => {
   mockDataResponse = JSON.stringify({message: 'Error thrown from Metro'});
   mockRequestError = null;
 
-  const error: mixed = await loadBundleFromServer(
+  const error: unknown = await loadBundleFromServer(
     '/Fail.bundle?platform=ios',
   ).catch(e => e);
 
@@ -112,7 +112,7 @@ test('loadBundleFromServer will throw LoadBundleFromServerError for request erro
   mockDataResponse = '';
   mockRequestError = 'Some error';
 
-  const error: mixed = await loadBundleFromServer(
+  const error: unknown = await loadBundleFromServer(
     '/Fail.bundle?platform=ios',
   ).catch(e => e);
 
diff --git a/packages/react-native/cli.js b/packages/react-native/cli.js
index 40abd7a1288c..2c6f6f779b22 100755
--- a/packages/react-native/cli.js
+++ b/packages/react-native/cli.js
@@ -47,7 +47,7 @@ function isMissingCliDependency(error /*: Error */) {
   );
 }
 
-let cli /*: $ReadOnly<{
+let cli /*: Readonly<{
   bin: string,
   loadConfig: $FlowFixMe,
   run: () => void
diff --git a/packages/react-native/react-native.config.js b/packages/react-native/react-native.config.js
index 2cb738b33534..452c5ffbf379 100644
--- a/packages/react-native/react-native.config.js
+++ b/packages/react-native/react-native.config.js
@@ -114,9 +114,9 @@ commands.push(codegenCommand);
 
 const config = {
   commands,
-  platforms: {} /*:: as {[string]: $ReadOnly<{
-      projectConfig: mixed,
-      dependencyConfig: mixed,
+  platforms: {} /*:: as {[string]: Readonly<{
+      projectConfig: unknown,
+      dependencyConfig: unknown,
     }>} */,
 };
 
diff --git a/packages/react-native/scripts/codegen/generate-artifacts-executor/generateCustomURLHandlers.js b/packages/react-native/scripts/codegen/generate-artifacts-executor/generateCustomURLHandlers.js
index 99f6c10ed20d..af9495e30440 100644
--- a/packages/react-native/scripts/codegen/generate-artifacts-executor/generateCustomURLHandlers.js
+++ b/packages/react-native/scripts/codegen/generate-artifacts-executor/generateCustomURLHandlers.js
@@ -26,7 +26,7 @@ const MODULES_PROTOCOLS_MM_TEMPLATE_PATH = path.join(
 );
 
 function generateCustomURLHandlers(
-  libraries /*: $ReadOnlyArray<$FlowFixMe> */,
+  libraries /*: ReadonlyArray<$FlowFixMe> */,
   outputDir /*: string */,
 ) {
   const iosAnnotations = parseiOSAnnotations(libraries);
diff --git a/packages/react-native/scripts/codegen/generate-artifacts-executor/generateNativeCode.js b/packages/react-native/scripts/codegen/generate-artifacts-executor/generateNativeCode.js
index 77d7c9505880..2f987b7d1ac7 100644
--- a/packages/react-native/scripts/codegen/generate-artifacts-executor/generateNativeCode.js
+++ b/packages/react-native/scripts/codegen/generate-artifacts-executor/generateNativeCode.js
@@ -19,7 +19,7 @@ const path = require('path');
 
 function generateNativeCode(
   outputPath /*: string */,
-  schemaInfos /*: $ReadOnlyArray<$FlowFixMe> */,
+  schemaInfos /*: ReadonlyArray<$FlowFixMe> */,
   includesGeneratedCode /*: boolean */,
   platform /*: string */,
   forceOutputPath /*: boolean */ = false,
diff --git a/packages/react-native/scripts/codegen/generate-artifacts-executor/generateRCTModuleProviders.js b/packages/react-native/scripts/codegen/generate-artifacts-executor/generateRCTModuleProviders.js
index b5b6ff52bfca..743861d57d16 100644
--- a/packages/react-native/scripts/codegen/generate-artifacts-executor/generateRCTModuleProviders.js
+++ b/packages/react-native/scripts/codegen/generate-artifacts-executor/generateRCTModuleProviders.js
@@ -32,7 +32,7 @@ const MODULE_PROVIDERS_MM_TEMPLATE_PATH = path.join(
 function generateRCTModuleProviders(
   projectRoot /*: string */,
   pkgJson /*: $FlowFixMe */,
-  libraries /*: $ReadOnlyArray<$FlowFixMe> */,
+  libraries /*: ReadonlyArray<$FlowFixMe> */,
   outputDir /*: string */,
 ) {
   fs.mkdirSync(outputDir, {recursive: true});
diff --git a/packages/react-native/scripts/codegen/generate-artifacts-executor/generateRCTThirdPartyComponents.js b/packages/react-native/scripts/codegen/generate-artifacts-executor/generateRCTThirdPartyComponents.js
index 1bbfd760debd..aab3783dbff8 100644
--- a/packages/react-native/scripts/codegen/generate-artifacts-executor/generateRCTThirdPartyComponents.js
+++ b/packages/react-native/scripts/codegen/generate-artifacts-executor/generateRCTThirdPartyComponents.js
@@ -31,7 +31,7 @@ const THIRD_PARTY_COMPONENTS_MM_TEMPLATE_PATH = path.join(
 );
 
 function generateRCTThirdPartyComponents(
-  libraries /*: $ReadOnlyArray<$FlowFixMe> */,
+  libraries /*: ReadonlyArray<$FlowFixMe> */,
   outputDir /*: string */,
 ) {
   fs.mkdirSync(outputDir, {recursive: true});
diff --git a/packages/react-native/scripts/codegen/generate-artifacts-executor/generateSchemaInfos.js b/packages/react-native/scripts/codegen/generate-artifacts-executor/generateSchemaInfos.js
index 68ec01a2d849..183626a8183f 100644
--- a/packages/react-native/scripts/codegen/generate-artifacts-executor/generateSchemaInfos.js
+++ b/packages/react-native/scripts/codegen/generate-artifacts-executor/generateSchemaInfos.js
@@ -17,7 +17,7 @@ const path = require('path');
 const {globSync} = require('tinyglobby');
 
 function generateSchemaInfos(
-  libraries /*: $ReadOnlyArray<$FlowFixMe> */,
+  libraries /*: ReadonlyArray<$FlowFixMe> */,
 ) /*: Array<$FlowFixMe> */ {
   // $FlowFixMe[incompatible-type]
   return libraries.map(generateSchemaInfo);
diff --git a/packages/react-native/scripts/codegen/generate-artifacts-executor/generateUnstableModulesRequiringMainQueueSetupProvider.js b/packages/react-native/scripts/codegen/generate-artifacts-executor/generateUnstableModulesRequiringMainQueueSetupProvider.js
index f2505a8f0145..31e2b437198c 100644
--- a/packages/react-native/scripts/codegen/generate-artifacts-executor/generateUnstableModulesRequiringMainQueueSetupProvider.js
+++ b/packages/react-native/scripts/codegen/generate-artifacts-executor/generateUnstableModulesRequiringMainQueueSetupProvider.js
@@ -28,7 +28,7 @@ const UNSTABLE_MODULES_REQUIRING_MAIN_QUEUE_SETUP_PROVIDER_MM_TEMPLATE_PATH =
   );
 
 function generateUnstableModulesRequiringMainQueueSetupProvider(
-  libraries /*: $ReadOnlyArray<$FlowFixMe> */,
+  libraries /*: ReadonlyArray<$FlowFixMe> */,
   outputDir /*: string */,
 ) {
   const iosAnnotations = parseiOSAnnotations(libraries);
diff --git a/packages/react-native/scripts/codegen/generate-artifacts-executor/utils.js b/packages/react-native/scripts/codegen/generate-artifacts-executor/utils.js
index 98ac2eb1c177..811fecb7a201 100644
--- a/packages/react-native/scripts/codegen/generate-artifacts-executor/utils.js
+++ b/packages/react-native/scripts/codegen/generate-artifacts-executor/utils.js
@@ -389,7 +389,7 @@ function isReactNativeCoreLibrary(libraryName /*: string */) /*: boolean */ {
  * Validates that components are defined in at most one library.
  */
 function parseiOSAnnotations(
-  libraries /*: $ReadOnlyArray<$FlowFixMe> */,
+  libraries /*: ReadonlyArray<$FlowFixMe> */,
 ) /*: {[string]: $FlowFixMe} */ {
   const mLibraryMap /*: {[string]: $FlowFixMe} */ = {};
   const cLibraryMap /*: {[string]: $FlowFixMe} */ = {};
diff --git a/packages/react-native/scripts/codegen/generate-specs-cli-executor.js b/packages/react-native/scripts/codegen/generate-specs-cli-executor.js
index e85e8361a2df..161ce0a2be7c 100644
--- a/packages/react-native/scripts/codegen/generate-specs-cli-executor.js
+++ b/packages/react-native/scripts/codegen/generate-specs-cli-executor.js
@@ -14,7 +14,7 @@ const utils = require('./codegen-utils');
 const fs = require('fs');
 const path = require('path');
 
-const GENERATORS /*: {[string]: {[string]: $ReadOnlyArray<string>}} */ = {
+const GENERATORS /*: {[string]: {[string]: ReadonlyArray<string>}} */ = {
   all: {
     android: ['componentsAndroid', 'modulesAndroid', 'modulesCxx'],
     ios: ['componentsIOS', 'modulesIOS', 'modulesCxx'],
diff --git a/packages/react-native/scripts/generate-provider-cli.js b/packages/react-native/scripts/generate-provider-cli.js
index 9324da65ae43..a3eb5cb36dde 100644
--- a/packages/react-native/scripts/generate-provider-cli.js
+++ b/packages/react-native/scripts/generate-provider-cli.js
@@ -34,7 +34,7 @@ const argv = yargs
     'Please provide platform, schema path, and output directory.',
   ).argv;
 
-const GENERATORS /*: {[string]: $ReadOnlyArray<string>} */ = {
+const GENERATORS /*: {[string]: ReadonlyArray<string>} */ = {
   android: [],
   ios: ['providerIOS'],
 };
diff --git a/packages/react-native/scripts/ios-prebuild/cli.js b/packages/react-native/scripts/ios-prebuild/cli.js
index 01301c800af7..1dfb23c23f24 100644
--- a/packages/react-native/scripts/ios-prebuild/cli.js
+++ b/packages/react-native/scripts/ios-prebuild/cli.js
@@ -14,7 +14,7 @@ const yargs = require('yargs');
 import type {BuildFlavor, Destination, Platform} from './types';
 */
 
-const platforms /*: $ReadOnlyArray<Platform> */ = [
+const platforms /*: ReadonlyArray<Platform> */ = [
   'ios',
   'ios-simulator',
   'mac-catalyst',
@@ -22,7 +22,7 @@ const platforms /*: $ReadOnlyArray<Platform> */ = [
 
 // CI can't use commas in cache keys, so 'macOS,variant=Mac Catalyst' was creating troubles
 // This map that converts from platforms to valid Xcodebuild destinations.
-const platformToDestination /*: $ReadOnly<{|[Platform]: Destination|}> */ = {
+const platformToDestination /*: Readonly<{|[Platform]: Destination|}> */ = {
   ios: 'iOS',
   'ios-simulator': 'iOS Simulator',
   'mac-catalyst': 'macOS,variant=Mac Catalyst',
@@ -78,7 +78,7 @@ async function getCLIConfiguration() /*: Promise<?{|
     compose: boolean,
   |},
   flavor: BuildFlavor,
-  destinations: $ReadOnlyArray<Destination>,
+  destinations: ReadonlyArray<Destination>,
   identity: ?string,
 |}> */ {
   // Run input parsing
diff --git a/packages/react-native/scripts/ios-prebuild/headers-config.js b/packages/react-native/scripts/ios-prebuild/headers-config.js
index 0d57d019e6e7..b5534b4054ef 100644
--- a/packages/react-native/scripts/ios-prebuild/headers-config.js
+++ b/packages/react-native/scripts/ios-prebuild/headers-config.js
@@ -9,12 +9,12 @@
  */
 
 /*::
-export type PodSpecConfiguration = $ReadOnly<{
+export type PodSpecConfiguration = Readonly<{
   name: string,
   headerPatterns: Array<string>,
   headerDir?: string,
   excludePatterns?: Array<string>,
-  subSpecs?: $ReadOnlyArray<PodSpecConfiguration>,
+  subSpecs?: ReadonlyArray<PodSpecConfiguration>,
   preservePaths?: Array<string>,
 } | {disabled: true}>;
 */
diff --git a/packages/react-native/src/private/renderer/branching/__tests__/ShadowTreeBranching-itest.js b/packages/react-native/src/private/renderer/branching/__tests__/ShadowTreeBranching-itest.js
index c2db162e2f51..25ea2e459e49 100644
--- a/packages/react-native/src/private/renderer/branching/__tests__/ShadowTreeBranching-itest.js
+++ b/packages/react-native/src/private/renderer/branching/__tests__/ShadowTreeBranching-itest.js
@@ -41,7 +41,7 @@ describe('ShadowTreeBranching', () => {
       'Insert {type: "View", parentNativeID: (root), index: 0, nativeID: "view"}',
     ]);
 
-    let updatesBeforeEndOfTask: $ReadOnlyArray<string> = [];
+    let updatesBeforeEndOfTask: ReadonlyArray<string> = [];
 
     Fantom.runTask(() => {
       root.render(
@@ -68,7 +68,7 @@ describe('ShadowTreeBranching', () => {
   it('should not skip intermediate React commits', () => {
     const root = Fantom.createRoot();
 
-    let updatesBeforeEndOfTask: $ReadOnlyArray<string> = [];
+    let updatesBeforeEndOfTask: ReadonlyArray<string> = [];
 
     // When rendered with `shouldTriggerIntermediate=false`, it renders two views
     // both with blue background. When rendered with `shouldTriggerIntermediate=true`,
diff --git a/packages/rn-tester/js/examples/AnimationBackend/SwipeableListExample.js b/packages/rn-tester/js/examples/AnimationBackend/SwipeableListExample.js
index 35e7d47aca6a..af0676bc2b7d 100644
--- a/packages/rn-tester/js/examples/AnimationBackend/SwipeableListExample.js
+++ b/packages/rn-tester/js/examples/AnimationBackend/SwipeableListExample.js
@@ -37,7 +37,7 @@ type Data = {
   title: string,
 };
 
-const initialData: $ReadOnlyArray<Data> = [
+const initialData: ReadonlyArray<Data> = [
   {id: '1', title: 'Kate Bell'},
   {id: '2', title: 'John Appleseed'},
   {id: '3', title: 'Mark Zuckerberg'},
@@ -62,7 +62,7 @@ const timingConfig = {
 };
 
 function SwipeableListExample(): React.Node {
-  const [data, setData] = React.useState<$ReadOnlyArray<Data>>(initialData);
+  const [data, setData] = React.useState<ReadonlyArray<Data>>(initialData);
 
   const handleRemove = useCallback((id: string) => {
     setData(currentData => currentData.filter(item => item.id !== id));
diff --git a/private/helloworld/metro.config.js b/private/helloworld/metro.config.js
index 543bfe6dd100..6f46f45d95c9 100644
--- a/private/helloworld/metro.config.js
+++ b/private/helloworld/metro.config.js
@@ -46,4 +46,4 @@ const config = {
 module.exports = mergeConfig(
   getDefaultConfig(__dirname),
   config,
-) /*:: as $ReadOnly<InputConfigT> */;
+) /*:: as Readonly<InputConfigT> */;
diff --git a/private/react-native-fantom/config/jest.config.js b/private/react-native-fantom/config/jest.config.js
index 34efa5467e77..1877182fff12 100644
--- a/private/react-native-fantom/config/jest.config.js
+++ b/private/react-native-fantom/config/jest.config.js
@@ -26,7 +26,7 @@ module.exports = {
     ...baseConfig.moduleFileExtensions,
     'cpp',
     'h',
-  ] /*:: as $ReadOnlyArray<string> */,
+  ] /*:: as ReadonlyArray<string> */,
   // This allows running Meta-internal tests with the `-test.fb.js` suffix.
   testRegex: '/__tests__/.*-itest(\\.fb)?\\.js$',
   testPathIgnorePatterns: [
diff --git a/private/react-native-fantom/runner/utils.js b/private/react-native-fantom/runner/utils.js
index b92125235b3a..11a30f1b212c 100644
--- a/private/react-native-fantom/runner/utils.js
+++ b/private/react-native-fantom/runner/utils.js
@@ -61,7 +61,7 @@ export function getBuckModesForPlatform({
 }: {
   enableCoverage: boolean,
   enableOptimized: boolean,
-}): $ReadOnlyArray<string> {
+}): ReadonlyArray<string> {
   let mode = enableCoverage ? 'code-coverage' : enableOptimized ? 'opt' : 'dev';
 
   if (enableOptimized) {
diff --git a/private/react-native-fantom/src/index.js b/private/react-native-fantom/src/index.js
index 85978d9669f4..72304f15d012 100644
--- a/private/react-native-fantom/src/index.js
+++ b/private/react-native-fantom/src/index.js
@@ -256,8 +256,9 @@ export function getDefinedEventHandlers(
   }
   // WARNING: This uses React private API (fiber internals).
   // $FlowExpectedError[incompatible-type]
-  const memoizedProps = (instanceHandle as {memoizedProps?: {[string]: mixed}})
-    .memoizedProps;
+  const memoizedProps = (
+    instanceHandle as {memoizedProps?: {[string]: unknown}}
+  ).memoizedProps;
   if (memoizedProps == null) {
     return [];
   }
diff --git a/scripts/build/build.js b/scripts/build/build.js
index c8b670a8c0e2..da2d642659c8 100644
--- a/scripts/build/build.js
+++ b/scripts/build/build.js
@@ -229,7 +229,7 @@ async function buildFile(
 /*::
 type PackageJson = {
   name: string,
-  exports?: {[subpath: string]: string | mixed},
+  exports?: {[subpath: string]: string | unknown},
 };
 */
 
diff --git a/scripts/debugger-frontend/sync-and-build.js b/scripts/debugger-frontend/sync-and-build.js
index db1f613bb3c8..72cc6ea6b1a6 100644
--- a/scripts/debugger-frontend/sync-and-build.js
+++ b/scripts/debugger-frontend/sync-and-build.js
@@ -163,9 +163,9 @@ async function buildDebuggerFrontend(
   packagePath /*: string */,
   scratchPath /*: string */,
   localCheckoutPath /*: ?string */,
-  {branch, gclientSyncOptions, noBuild} /*: $ReadOnly<{
+  {branch, gclientSyncOptions, noBuild} /*: Readonly<{
     branch: string,
-    gclientSyncOptions: $ReadOnly<{nohooks: boolean}>,
+    gclientSyncOptions: Readonly<{nohooks: boolean}>,
     noBuild: boolean,
   }>*/,
 ) /*: Promise<{checkoutPath: string}> */ {
@@ -244,7 +244,7 @@ async function checkoutDevToolsFrontend(
 async function setupGclientWorkspace(
   scratchPath /*: string */,
   checkoutPath /*: string */,
-  {nohooks} /*: $ReadOnly<{nohooks: boolean}> */,
+  {nohooks} /*: Readonly<{nohooks: boolean}> */,
 ) {
   process.stdout.write('Setting up gclient workspace' + '\n');
   await spawnSafe(
@@ -345,12 +345,12 @@ async function copyLicenseToPackage(
 }
 
 async function generateBuildInfo(
-  info /*: $ReadOnly<{
+  info /*: Readonly<{
   checkoutPath: string,
   isLocalCheckout: boolean,
   branch: string,
   packagePath: string,
-  gclientSyncOptions: $ReadOnly<{nohooks: boolean}>,
+  gclientSyncOptions: Readonly<{nohooks: boolean}>,
   gnArgsSummary: string,
   noBuild: boolean,
 }> */,
@@ -588,7 +588,7 @@ async function createSyncDiff(
   {
     checkoutPath,
     noBuild,
-  } /*: $ReadOnly<{checkoutPath: string, noBuild: boolean}> */,
+  } /*: Readonly<{checkoutPath: string, noBuild: boolean}> */,
 ) {
   process.stdout.write('Creating a sync diff\n');
   const {packagePath, baseGitRevision} = diffBaseInfo;
diff --git a/scripts/debugger-shell/build-binary.js b/scripts/debugger-shell/build-binary.js
index cb34bbffa502..e7300579b1c7 100644
--- a/scripts/debugger-shell/build-binary.js
+++ b/scripts/debugger-shell/build-binary.js
@@ -16,7 +16,7 @@ try {
   isMetaInternal = false;
 }
 
-let additionalConfig /*: $ReadOnly<{
+let additionalConfig /*: Readonly<{
   appVersionHash: ?string,
 }> */ = {
   appVersionHash: null,
diff --git a/scripts/e2e/utils/try-n-times.js b/scripts/e2e/utils/try-n-times.js
index ac6fa759a816..904e44f1f3a0 100644
--- a/scripts/e2e/utils/try-n-times.js
+++ b/scripts/e2e/utils/try-n-times.js
@@ -15,7 +15,7 @@
 function tryExecNTimes(
   funcToRetry /*: () => number */,
   retriesLeft /*: number */,
-  onEveryError /*: ?(() => mixed) */,
+  onEveryError /*: ?(() => unknown) */,
 ) /*: number */ {
   const exitCode = funcToRetry();
   if (exitCode === 0) {
diff --git a/scripts/releases/ios-prebuild/build.js b/scripts/releases/ios-prebuild/build.js
index bb3e33ff5934..3a913e3ae46b 100644
--- a/scripts/releases/ios-prebuild/build.js
+++ b/scripts/releases/ios-prebuild/build.js
@@ -22,8 +22,8 @@ import type { Dependency, Destination, Platform } from './types';
 async function buildDepenencies(
   scheme /*: string */,
   configuration /*: string */,
-  dependencies /*: $ReadOnlyArray<Dependency> */,
-  destinations /*: $ReadOnlyArray<Destination> */,
+  dependencies /*: ReadonlyArray<Dependency> */,
+  destinations /*: ReadonlyArray<Destination> */,
   rootFolder /*: string */,
   buildFolder /*: string */,
 ) {
diff --git a/scripts/releases/ios-prebuild/cli.js b/scripts/releases/ios-prebuild/cli.js
index dc0a7f7b7b74..dffca8203350 100644
--- a/scripts/releases/ios-prebuild/cli.js
+++ b/scripts/releases/ios-prebuild/cli.js
@@ -17,7 +17,7 @@ import type {Dependency, Destination, Platform} from './types';
 
 // CI can't use commas in cache keys, so 'macOS,variant=Mac Catalyst' was creating troubles
 // This map that converts from platforms to valid Xcodebuild destinations.
-const platformToDestination /*: $ReadOnly<{|[Platform]: Destination|}> */ = {
+const platformToDestination /*: Readonly<{|[Platform]: Destination|}> */ = {
   ios: 'iOS',
   'ios-simulator': 'iOS Simulator',
   macos: 'macOS',
@@ -92,8 +92,8 @@ async function getCLIConfiguration() /*: Promise<?{|
     build: boolean,
     compose: boolean,
   |},
-  destinations: $ReadOnlyArray<Destination>,
-  dependencies: $ReadOnlyArray<Dependency>,
+  destinations: ReadonlyArray<Destination>,
+  dependencies: ReadonlyArray<Dependency>,
   configuration: string,
   identity: ?string,
 |}> */ {
diff --git a/scripts/releases/ios-prebuild/compose-framework.js b/scripts/releases/ios-prebuild/compose-framework.js
index 599622af6bc4..fd0e1287d1f3 100644
--- a/scripts/releases/ios-prebuild/compose-framework.js
+++ b/scripts/releases/ios-prebuild/compose-framework.js
@@ -23,7 +23,7 @@ import type { Dependency, Platform } from './types';
 async function createFramework(
   scheme /*: string */,
   configuration /*: string */,
-  dependencies /*: $ReadOnlyArray<Dependency> */,
+  dependencies /*: ReadonlyArray<Dependency> */,
   rootFolder /*: string */,
   buildFolder /*: string */,
   identity /*: ?string */,
@@ -81,7 +81,7 @@ async function createFramework(
  */
 function copyHeaders(
   scheme /*: string */,
-  dependencies /*: $ReadOnlyArray<Dependency> */,
+  dependencies /*: ReadonlyArray<Dependency> */,
   rootFolder /*: string */,
 ) {
   console.log('Copying header files for dependencies...');
@@ -114,7 +114,7 @@ function copyHeaders(
  */
 function copyBundles(
   scheme /*: string */,
-  dependencies /*: $ReadOnlyArray<Dependency> */,
+  dependencies /*: ReadonlyArray<Dependency> */,
   outputFolder /*:string*/,
   frameworkPaths /*:Array<string>*/,
 ) {
diff --git a/scripts/releases/ios-prebuild/configuration.js b/scripts/releases/ios-prebuild/configuration.js
index 4a00cdc0e4b4..8bf6f5db12aa 100644
--- a/scripts/releases/ios-prebuild/configuration.js
+++ b/scripts/releases/ios-prebuild/configuration.js
@@ -12,7 +12,7 @@
 import type { Dependency, Platform } from './types';
 */
 
-const platforms /*: $ReadOnlyArray<Platform> */ = [
+const platforms /*: ReadonlyArray<Platform> */ = [
   'ios',
   'ios-simulator',
   'macos',
@@ -25,7 +25,7 @@ const platforms /*: $ReadOnlyArray<Platform> */ = [
 
 const CPP_STANDARD = 'c++20';
 
-const dependencies /*: $ReadOnlyArray<Dependency> */ = [
+const dependencies /*: ReadonlyArray<Dependency> */ = [
   {
     name: 'glog',
     version: '0.3.5',
diff --git a/scripts/releases/ios-prebuild/setupDependencies.js b/scripts/releases/ios-prebuild/setupDependencies.js
index 152f2af920f3..07a1b668997a 100644
--- a/scripts/releases/ios-prebuild/setupDependencies.js
+++ b/scripts/releases/ios-prebuild/setupDependencies.js
@@ -32,7 +32,7 @@ const exec = util.promisify(require('child_process').exec);
  * according to the configuration provided.
  */
 async function setupDependencies(
-  dependencies /*: $ReadOnlyArray<Dependency> */,
+  dependencies /*: ReadonlyArray<Dependency> */,
   rootFolder /*: string */,
 ) {
   console.log('✅ Setting up dependencies...');
diff --git a/scripts/releases/ios-prebuild/swift-package.js b/scripts/releases/ios-prebuild/swift-package.js
index 269c52e9cb37..68ca681b2dfc 100644
--- a/scripts/releases/ios-prebuild/swift-package.js
+++ b/scripts/releases/ios-prebuild/swift-package.js
@@ -21,7 +21,7 @@ const path = require('path');
  */
 async function createSwiftPackageFile(
   scheme /*: string */,
-  dependencies /*  :$ReadOnlyArray<Dependency> */,
+  dependencies /*  :ReadonlyArray<Dependency> */,
   rootFolder /*: string */,
 ) {
   console.log(
diff --git a/scripts/releases/ios-prebuild/types.js b/scripts/releases/ios-prebuild/types.js
index c51574a82650..8829b2553e69 100644
--- a/scripts/releases/ios-prebuild/types.js
+++ b/scripts/releases/ios-prebuild/types.js
@@ -11,10 +11,10 @@
 /*::
 export type Folder = RegExp;
 
-export type Files = $ReadOnly<{
-  headers: $ReadOnlyArray<string>,
-  sources: $ReadOnlyArray<string>,
-  resources?: $ReadOnlyArray<string>,
+export type Files = Readonly<{
+  headers: ReadonlyArray<string>,
+  sources: ReadonlyArray<string>,
+  resources?: ReadonlyArray<string>,
   // Relative path from target root to where the header files should be copied.
   // Can be used to ensure header search paths like <double-conversion/double-conversion.h>
   // are correctly resolved.
@@ -22,22 +22,22 @@ export type Files = $ReadOnly<{
   headerSkipFolderNames?: string,
 }>;
 
-export type Define = $ReadOnly<{
+export type Define = Readonly<{
   name: string,
   value?: string,
 }>;
 
-export type Settings = $ReadOnly<{
-  headerSearchPaths?: $ReadOnlyArray<string>,
-  defines?: $ReadOnlyArray<Define>,
-  cCompilerFlags?: $ReadOnlyArray<string>,
-  cxxCompilerFlags?: $ReadOnlyArray<string>,
-  linkedLibraries?: $ReadOnlyArray<string>,
+export type Settings = Readonly<{
+  headerSearchPaths?: ReadonlyArray<string>,
+  defines?: ReadonlyArray<Define>,
+  cCompilerFlags?: ReadonlyArray<string>,
+  cxxCompilerFlags?: ReadonlyArray<string>,
+  linkedLibraries?: ReadonlyArray<string>,
   publicHeaderFiles: string,
-  linkerSettings?: $ReadOnlyArray<string>
+  linkerSettings?: ReadonlyArray<string>
 }>;
 
-export type Dependency = $ReadOnly<{
+export type Dependency = Readonly<{
   name: string,
   version: string,
   url: URL,
@@ -45,7 +45,7 @@ export type Dependency = $ReadOnly<{
   files: Files,
   settings: Settings,
   disabled?: boolean,
-  dependencies?: $ReadOnlyArray<string>,
+  dependencies?: ReadonlyArray<string>,
 }>;
 
 export type Platform =
diff --git a/scripts/releases/upload-release-assets-for-dotslash.js b/scripts/releases/upload-release-assets-for-dotslash.js
index 3fa0000f2508..270d265909b0 100644
--- a/scripts/releases/upload-release-assets-for-dotslash.js
+++ b/scripts/releases/upload-release-assets-for-dotslash.js
@@ -30,15 +30,15 @@ import type {DotSlashProvider, DotSlashHttpProvider, DotSlashArtifactInfo} from
 import type {IOctokit} from './utils/octokit-utils';
 
 type GitHubReleaseAsset = {id: string, ...};
-type ReleaseAssetMap = $ReadOnlyMap<string, GitHubReleaseAsset>;
+type ReleaseAssetMap = ReadonlyMap<string, GitHubReleaseAsset>;
 
-type ReleaseInfo = $ReadOnly<{
+type ReleaseInfo = Readonly<{
   releaseId: string,
   releaseTag: string,
   existingAssetsByName: ReleaseAssetMap,
 }>;
 
-type ExecutionOptions = $ReadOnly<{
+type ExecutionOptions = Readonly<{
   force: boolean,
   dryRun: boolean,
 }>;
@@ -191,7 +191,7 @@ async function fetchUpstreamAssetAndUploadToRelease(
     artifactInfo,
     dotslashFilename,
   } /*: {
-  providers: $ReadOnlyArray<DotSlashProvider>,
+  providers: ReadonlyArray<DotSlashProvider>,
   suggestedFilename: string,
   artifactInfo: DotSlashArtifactInfo,
   dotslashFilename: string,
diff --git a/scripts/releases/utils/dotslash-utils.js b/scripts/releases/utils/dotslash-utils.js
index eb435fffd71f..238da891ab9f 100644
--- a/scripts/releases/utils/dotslash-utils.js
+++ b/scripts/releases/utils/dotslash-utils.js
@@ -40,7 +40,7 @@ type DotSlashPlatformSpec = {
   ...
 };
 
-export type DotSlashArtifactInfo = $ReadOnly<{
+export type DotSlashArtifactInfo = Readonly<{
   size: number,
   hash: 'blake3' | 'sha256',
   digest: string,
@@ -54,13 +54,13 @@ type JSONCFormattingOptions = {
 };
 
 type DotSlashProvidersTransformFn = (
-  providers: $ReadOnlyArray<DotSlashProvider>,
+  providers: ReadonlyArray<DotSlashProvider>,
   suggestedFilename: string,
   artifactInfo: DotSlashArtifactInfo,
-) => ?$ReadOnlyArray<DotSlashProvider> | Promise<?$ReadOnlyArray<DotSlashProvider>>;
+) => ?ReadonlyArray<DotSlashProvider> | Promise<?ReadonlyArray<DotSlashProvider>>;
 */
 
-const DEFAULT_FORMATTING_OPTIONS /*: $ReadOnly<JSONCFormattingOptions> */ = {
+const DEFAULT_FORMATTING_OPTIONS /*: Readonly<JSONCFormattingOptions> */ = {
   tabSize: 4,
   insertSpaces: true,
   eol: '\n',
@@ -75,7 +75,7 @@ const DEFAULT_FORMATTING_OPTIONS /*: $ReadOnly<JSONCFormattingOptions> */ = {
 async function processDotSlashFileInPlace(
   filename /*: string */,
   transformProviders /*: DotSlashProvidersTransformFn */,
-  formattingOptions /*: $ReadOnly<JSONCFormattingOptions> */ = DEFAULT_FORMATTING_OPTIONS,
+  formattingOptions /*: Readonly<JSONCFormattingOptions> */ = DEFAULT_FORMATTING_OPTIONS,
 ) /*: Promise<void> */ {
   // Validate the file using `dotslash` itself so we can be reasonably sure that it conforms
   // to the expected format.
@@ -87,7 +87,7 @@ async function processDotSlashFileInPlace(
   const json = parse(originalContentsJson);
   let intermediateContentsJson = originalContentsJson;
   for (const [platform, platformSpec] of Object.entries(json.platforms) /*::
-   as $ReadOnlyArray<[string, DotSlashPlatformSpec]>
+   as ReadonlyArray<[string, DotSlashPlatformSpec]>
   */) {
     const providers = platformSpec.providers;
     const suggestedFilename =
@@ -144,7 +144,7 @@ function splitShebangFromContents(
  */
 async function validateAndParseDotSlashFile(
   filename /*: string */,
-) /*: mixed */ {
+) /*: unknown */ {
   const {stdout} = await execFile(dotslash, ['--', 'parse', filename]);
   return JSON.parse(stdout);
 }
diff --git a/scripts/releases/write-dotslash-release-asset-urls.js b/scripts/releases/write-dotslash-release-asset-urls.js
index a67eff51e4c3..2e7a650dc900 100644
--- a/scripts/releases/write-dotslash-release-asset-urls.js
+++ b/scripts/releases/write-dotslash-release-asset-urls.js
@@ -99,7 +99,7 @@ async function writeReleaseAssetUrlsToDotSlashFile(
 
 async function updateAndVerifyProviders(
   {providers: providersArg, suggestedFilename, artifactInfo, releaseTag} /*:
-  {providers: $ReadOnlyArray<DotSlashProvider>,
+  {providers: ReadonlyArray<DotSlashProvider>,
   suggestedFilename: string,
   artifactInfo: DotSlashArtifactInfo,
   releaseTag: string,}
@@ -151,8 +151,8 @@ function createReleaseAssetProvider(
 }
 
 function diffProviderArrays(
-  original /*: $ReadOnlyArray<DotSlashProvider> */,
-  updated /*: $ReadOnlyArray<DotSlashProvider> */,
+  original /*: ReadonlyArray<DotSlashProvider> */,
+  updated /*: ReadonlyArray<DotSlashProvider> */,
 ) {
   return jestDiff(original, updated, {
     aAnnotation: 'Original',
diff --git a/scripts/run-ci-javascript-tests.js b/scripts/run-ci-javascript-tests.js
index ba8f95384dde..8e62419ba971 100644
--- a/scripts/run-ci-javascript-tests.js
+++ b/scripts/run-ci-javascript-tests.js
@@ -20,7 +20,7 @@
  */
 
 const {execSync} = require('child_process');
-const argv /*:$ReadOnly<{
+const argv /*:Readonly<{
   maxWorkers?: number,
   jestBinary?: string,
   flowBinary?: string,
diff --git a/scripts/shared/monorepoUtils.js b/scripts/shared/monorepoUtils.js
index f61803bfaaae..6260b0a19f70 100644
--- a/scripts/shared/monorepoUtils.js
+++ b/scripts/shared/monorepoUtils.js
@@ -27,7 +27,7 @@ export type PackageJson = {
   ...
 };
 
-type PackagesFilter = $ReadOnly<{
+type PackagesFilter = Readonly<{
   // Include the main react-native package
   includeReactNative: boolean,
 
@@ -135,7 +135,7 @@ async function parsePackageInfo(
  */
 async function updatePackageJson(
   {path: packagePath, packageJson} /*: PackageInfo */,
-  newPackageVersions /*: $ReadOnly<{[string]: string}> */,
+  newPackageVersions /*: Readonly<{[string]: string}> */,
 ) /*: Promise<void> */ {
   const packageName = packageJson.name;