diff --git a/packages/metro-config/API.md b/packages/metro-config/API.md
index 6f2d251d24..553483f022 100644
--- a/packages/metro-config/API.md
+++ b/packages/metro-config/API.md
@@ -179,6 +179,7 @@ export type SerializerConfigT = {
     processModuleFilter: (modules: Module) => boolean;
     isThirdPartyModule: (module: Readonly<{path: string}>) => boolean;
     unstable_inlineDependencyMap: boolean;
+    unstable_lazilyDefineModules: boolean;
 };
 
 export type ServerConfigT = {
diff --git a/packages/metro-config/src/defaults/index.js b/packages/metro-config/src/defaults/index.js
index 24507326b0..3da8b3b439 100644
--- a/packages/metro-config/src/defaults/index.js
+++ b/packages/metro-config/src/defaults/index.js
@@ -71,6 +71,7 @@ const getDefaultValues = (projectRoot: ?string): ConfigT => ({
     isThirdPartyModule: module =>
       /(?:^|[/\\])node_modules[/\\]/.test(module.path),
     unstable_inlineDependencyMap: false,
+    unstable_lazilyDefineModules: false,
   },
 
   server: {
diff --git a/packages/metro-config/src/types.js b/packages/metro-config/src/types.js
index 8b0480500f..7678e4e1ab 100644
--- a/packages/metro-config/src/types.js
+++ b/packages/metro-config/src/types.js
@@ -148,6 +148,12 @@ type SerializerConfigT = {
   processModuleFilter: (modules: Module<>) => boolean,
   isThirdPartyModule: (module: Readonly<{path: string, ...}>) => boolean,
   unstable_inlineDependencyMap: boolean,
+  // When true, the default bundle serializer emits modules inside a single
+  // segment definer (`__registerSegment(0, function (moduleId) { switch ... })`)
+  // so each `__d(...)` runs lazily on first require instead of eagerly at
+  // startup. Reduces startup registration cost and peak heap for large graphs.
+  // Experimental; source maps are supported. Does not affect delta/HMR.
+  unstable_lazilyDefineModules: boolean,
 };
 
 type TransformerConfigT = {
diff --git a/packages/metro/src/DeltaBundler/Serializers/__tests__/baseJSBundleWithLazyModuleDefinition-test.js b/packages/metro/src/DeltaBundler/Serializers/__tests__/baseJSBundleWithLazyModuleDefinition-test.js
new file mode 100644
index 0000000000..e4971b1952
--- /dev/null
+++ b/packages/metro/src/DeltaBundler/Serializers/__tests__/baseJSBundleWithLazyModuleDefinition-test.js
@@ -0,0 +1,207 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow strict-local
+ * @format
+ * @oncall react_native
+ */
+
+import type {Module, TransformInputOptions} from '../../types';
+
+import CountingSet from '../../../lib/CountingSet';
+import baseJSBundleWithLazyModuleDefinition from '../baseJSBundleWithLazyModuleDefinition';
+
+const polyfill: Module<> = {
+  path: '/polyfill',
+  dependencies: new Map(),
+  inverseDependencies: new CountingSet(),
+  output: [
+    {
+      type: 'js/script',
+      data: {code: '__d(function() {/* code for polyfill */});', lineCount: 1},
+    },
+  ],
+  getSource: () => Buffer.from('polyfill-source'),
+};
+
+const fooModule: Module<> = {
+  path: '/root/foo',
+  dependencies: new Map([
+    [
+      './bar',
+      {
+        absolutePath: '/root/bar',
+        data: {
+          data: {asyncType: null, isESMImport: false, locs: [], key: './bar'},
+          name: './bar',
+        },
+      },
+    ],
+  ]),
+  inverseDependencies: new CountingSet(),
+  output: [
+    {
+      type: 'js/module',
+      data: {
+        code: '__d(function() {/* code for foo */});',
+        map: [],
+        lineCount: 1,
+      },
+    },
+  ],
+  getSource: () => Buffer.from('foo-source'),
+};
+
+const barModule: Module<> = {
+  path: '/root/bar',
+  dependencies: new Map(),
+  inverseDependencies: new CountingSet(['/root/foo']),
+  output: [
+    {
+      type: 'js/module',
+      data: {
+        code: '__d(function() {/* code for bar */});',
+        map: [],
+        lineCount: 1,
+      },
+    },
+  ],
+  getSource: () => Buffer.from('bar-source'),
+};
+
+const transformOptions: TransformInputOptions = {
+  customTransformOptions: {},
+  dev: true,
+  minify: true,
+  platform: 'web',
+  type: 'module',
+  unstable_transformProfile: 'default',
+};
+
+function serialize() {
+  return baseJSBundleWithLazyModuleDefinition(
+    '/root/foo',
+    [polyfill],
+    {
+      dependencies: new Map([
+        ['/root/foo', fooModule],
+        ['/root/bar', barModule],
+      ]),
+      entryPoints: new Set(['/root/foo']),
+      transformOptions,
+    },
+    {
+      asyncRequireModulePath: '',
+      createModuleId: (filePath: string) => (filePath === '/root/foo' ? 0 : 1),
+      dev: true,
+      getRunModuleStatement: (moduleId: number | string) =>
+        `require(${JSON.stringify(moduleId)});`,
+      globalPrefix: '',
+      includeAsyncPaths: false,
+      inlineSourceMap: false,
+      modulesOnly: false,
+      processModuleFilter: () => true,
+      projectRoot: '/root',
+      runBeforeMainModule: [],
+      runModule: true,
+      serverRoot: '/root',
+      shouldAddToIgnoreList: () => false,
+      sourceMapUrl: 'http://localhost/bundle.map',
+      sourceUrl: null,
+      getSourceUrl: null,
+    },
+  );
+}
+
+test('wraps real modules in a segment switch and emits polyfills eagerly', () => {
+  const {code} = serialize();
+
+  // Polyfill is emitted eagerly at the top.
+  expect(code).toContain('__d(function() {/* code for polyfill */});');
+
+  // Real modules are registered lazily via a single segment definer.
+  expect(code).toContain('__registerSegment(0, function defSeg0(moduleId) {');
+  expect(code).toContain('var ___d = __d;');
+  expect(code).toContain('switch (moduleId) {');
+  expect(code).toContain('case 0:');
+  expect(code).toContain('case 1:');
+  expect(code).toContain('___d(function() {/* code for foo */}');
+  expect(code).toContain('return;');
+  expect(code).toContain(
+    'default: new Error("No module found for ID " + moduleId);',
+  );
+
+  // Run-module call comes after registration.
+  expect(code).toContain('require(0);');
+});
+
+test('polyfills lead, modules are inside the switch, run-module trails', () => {
+  const {code} = serialize();
+  const polyfillAt = code.indexOf('code for polyfill');
+  const segmentAt = code.indexOf('__registerSegment');
+  const fooAt = code.indexOf('code for foo');
+  const runAt = code.indexOf('require(0);');
+
+  expect(polyfillAt).toBeGreaterThanOrEqual(0);
+  expect(segmentAt).toBeGreaterThan(polyfillAt);
+  // foo's __d is inside the switch (after __registerSegment), not eager.
+  expect(fooAt).toBeGreaterThan(segmentAt);
+  expect(runAt).toBeGreaterThan(segmentAt);
+});
+
+test('produces a valid indexed source map', () => {
+  const {map} = serialize();
+  const parsed = JSON.parse(map);
+  expect(parsed.version).toBe(3);
+  // BundleBuilder emits a sectioned (indexed) map.
+  expect(Array.isArray(parsed.sections)).toBe(true);
+  expect(parsed.sections.length).toBeGreaterThan(0);
+  for (const section of parsed.sections) {
+    expect(section.offset).toEqual(
+      expect.objectContaining({
+        line: expect.any(Number),
+        column: expect.any(Number),
+      }),
+    );
+  }
+});
+
+test('modulesOnly omits the eager polyfills but keeps the segment', () => {
+  const {code} = baseJSBundleWithLazyModuleDefinition(
+    '/root/foo',
+    [polyfill],
+    {
+      dependencies: new Map([['/root/foo', fooModule]]),
+      entryPoints: new Set(['/root/foo']),
+      transformOptions,
+    },
+    {
+      asyncRequireModulePath: '',
+      createModuleId: () => 0,
+      dev: true,
+      getRunModuleStatement: (moduleId: number | string) =>
+        `require(${JSON.stringify(moduleId)});`,
+      globalPrefix: '',
+      includeAsyncPaths: false,
+      inlineSourceMap: false,
+      modulesOnly: true,
+      processModuleFilter: () => true,
+      projectRoot: '/root',
+      runBeforeMainModule: [],
+      runModule: true,
+      serverRoot: '/root',
+      shouldAddToIgnoreList: () => false,
+      sourceMapUrl: null,
+      sourceUrl: null,
+      getSourceUrl: null,
+    },
+  );
+
+  expect(code).not.toContain('code for polyfill');
+  expect(code).toContain('__registerSegment(0, function defSeg0(moduleId) {');
+  expect(code).toContain('var ___d = __d;');
+  expect(code).toContain('case 0:');
+});
diff --git a/packages/metro/src/DeltaBundler/Serializers/baseJSBundleWithLazyModuleDefinition.js b/packages/metro/src/DeltaBundler/Serializers/baseJSBundleWithLazyModuleDefinition.js
new file mode 100644
index 0000000000..7365f1ec1b
--- /dev/null
+++ b/packages/metro/src/DeltaBundler/Serializers/baseJSBundleWithLazyModuleDefinition.js
@@ -0,0 +1,157 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow strict-local
+ * @format
+ * @oncall react_native
+ */
+
+import type {Module, ReadOnlyGraph, SerializerOptions} from '../types';
+import type {LazyModuleSwitchEntry} from './helpers/lazyModuleSwitch';
+
+import getAppendScripts from '../../lib/getAppendScripts';
+import getSourceMapInfo from './helpers/getSourceMapInfo';
+import {isJsModule, wrapModule} from './helpers/js';
+import {appendLazyModuleSwitch} from './helpers/lazyModuleSwitch';
+import processModules from './helpers/processModules';
+import {BundleBuilder, fromRawMappings} from 'metro-source-map';
+
+type Options = Readonly<{
+  ...SerializerOptions,
+  excludeSource?: boolean,
+}>;
+
+/**
+ * Serializes a bundle in which the graph's modules are placed inside a single
+ * segment definer -- `__registerSegment(0, function (moduleId) { switch (...) })`
+ * -- so that each module's `__d(...)` call runs lazily on first require instead
+ * of eagerly at startup. Polyfills and the require runtime (pre-modules) and the
+ * run-module calls (append scripts) are emitted eagerly, as usual.
+ *
+ * This is the OSS equivalent of the metro-buck "plain bundle with switch"
+ * output, gated behind `serializer.unstable_lazilyDefineModules`. Unlike
+ * `baseJSBundle` + `bundleToString`, which keep modules as independently
+ * addressable top-level `__d(...)` statements, the switch form wraps them all in
+ * one function -- so the code and source map must be assembled together (a
+ * `BundleBuilder`), rather than via the flat index-map path which assumes the
+ * plain layout.
+ *
+ * The structured graph is untouched, so deltas and HMR (which address modules
+ * individually via `hmrJSBundle`) are unaffected: an HMR update is still a
+ * top-level `__d(...)` that shadows the switch branch. The runtime materialises
+ * a not-yet-required module on demand via its segment definer (see
+ * `ensureModuleRegistered` in the require polyfill), so Fast Refresh keeps
+ * working with behaviour identical to an eager bundle.
+ */
+export default function baseJSBundleWithLazyModuleDefinition(
+  entryPoint: string,
+  preModules: ReadonlyArray<Module<>>,
+  graph: ReadOnlyGraph<>,
+  options: Options,
+): {code: string, map: string} {
+  // Assign ids up front so ordering and dependency-map ids are stable, matching
+  // `baseJSBundle`.
+  for (const module of graph.dependencies.values()) {
+    options.createModuleId(module.path);
+  }
+
+  const excludeSource = options.excludeSource === true;
+
+  const wrapOptions = {
+    createModuleId: options.createModuleId,
+    dev: options.dev,
+    includeAsyncPaths: options.includeAsyncPaths,
+    projectRoot: options.projectRoot,
+    serverRoot: options.serverRoot,
+    sourceUrl: options.sourceUrl,
+    dependencyMapReservedName: options.dependencyMapReservedName,
+    unstable_inlineDependencyMap: options.unstable_inlineDependencyMap,
+    unstable_getAsyncDependencyPath: options.unstable_getAsyncDependencyPath,
+  };
+
+  const mapOptions = {
+    excludeSource,
+    shouldAddToIgnoreList: options.shouldAddToIgnoreList,
+    getSourceUrl: options.getSourceUrl,
+  };
+
+  const builder = new BundleBuilder(options.sourceUrl ?? 'bundle.js');
+
+  const getModuleEntry = (module: Module<>): LazyModuleSwitchEntry => {
+    const code = wrapModule(module, wrapOptions);
+    const info = getSourceMapInfo(module, mapOptions);
+    // Per-module map in its own coordinate space; `BundleBuilder` offsets each
+    // section by the current output position, so the wrapping (`case N:` prefix,
+    // segment preamble, preceding modules) is accounted for automatically.
+    const map = fromRawMappings([info]).toMap(undefined, {excludeSource});
+    return {
+      code,
+      map,
+      moduleId: options.createModuleId(module.path),
+      sourcePath: module.path,
+    };
+  };
+
+  const appendModule = (module: Module<>): void => {
+    const entry = getModuleEntry(module);
+    builder.append(entry.code, entry.map);
+  };
+
+  // Pre-modules (polyfills + require runtime) are emitted eagerly at the top.
+  if (!options.modulesOnly) {
+    for (const module of preModules) {
+      if (isJsModule(module) && options.processModuleFilter(module)) {
+        appendModule(module);
+        builder.append('\n');
+      }
+    }
+  }
+
+  const modules = [...graph.dependencies.values()]
+    .filter(isJsModule)
+    .filter(options.processModuleFilter)
+    .sort(
+      (a, b) => options.createModuleId(a.path) - options.createModuleId(b.path),
+    );
+
+  builder.append('__registerSegment(0, function defSeg0(moduleId) {');
+  appendLazyModuleSwitch(builder, modules.map(getModuleEntry), {
+    globalPrefix: options.globalPrefix,
+  });
+  builder.append('});\n');
+
+  // Run-module calls (and the trailing sourceMappingURL comment) stay eager,
+  // after registration.
+  const postScripts = processModules(
+    getAppendScripts(entryPoint, [...preModules, ...modules], {
+      asyncRequireModulePath: options.asyncRequireModulePath,
+      createModuleId: options.createModuleId,
+      getRunModuleStatement: options.getRunModuleStatement,
+      globalPrefix: options.globalPrefix,
+      inlineSourceMap: options.inlineSourceMap,
+      runBeforeMainModule: options.runBeforeMainModule,
+      runModule: options.runModule,
+      shouldAddToIgnoreList: options.shouldAddToIgnoreList,
+      sourceMapUrl: options.sourceMapUrl,
+      sourceUrl: options.sourceUrl,
+      getSourceUrl: options.getSourceUrl,
+    }),
+    {
+      filter: options.processModuleFilter,
+      ...wrapOptions,
+    },
+  );
+  for (const [, code] of postScripts) {
+    if (code.length > 0) {
+      builder.append(code + '\n');
+    }
+  }
+
+  return {
+    code: builder.getCode(),
+    map: JSON.stringify(builder.getMap()),
+  };
+}
diff --git a/packages/metro/src/DeltaBundler/Serializers/helpers/lazyModuleSwitch.js b/packages/metro/src/DeltaBundler/Serializers/helpers/lazyModuleSwitch.js
new file mode 100644
index 0000000000..799b6f620f
--- /dev/null
+++ b/packages/metro/src/DeltaBundler/Serializers/helpers/lazyModuleSwitch.js
@@ -0,0 +1,53 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow strict-local
+ * @format
+ * @oncall react_native
+ */
+
+import type {BasicSourceMap, BundleBuilder} from 'metro-source-map';
+
+export type LazyModuleSwitchEntry = Readonly<{
+  moduleId: number | string,
+  code: string,
+  map: ?BasicSourceMap,
+  sourcePath: string,
+}>;
+
+export function appendLazyModuleSwitch(
+  builder: BundleBuilder,
+  entries: ReadonlyArray<LazyModuleSwitchEntry>,
+  options: Readonly<{globalPrefix: string}>,
+): void {
+  const modulePrefix = '_';
+
+  builder.append(
+    `var ${modulePrefix}${options.globalPrefix}__d = ${options.globalPrefix}__d;`,
+  );
+  builder.append('switch (moduleId) {\n');
+
+  for (const entry of entries) {
+    builder
+      .append(
+        'case ' + entry.moduleId + ':\n' + modulePrefix,
+        moduleAnchorMap(entry.sourcePath),
+      )
+      .append(entry.code, entry.map)
+      .append('return;\n');
+  }
+
+  builder.append('default: new Error("No module found for ID " + moduleId);}');
+}
+
+function moduleAnchorMap(sourcePath: string): BasicSourceMap {
+  return {
+    version: 3,
+    sources: [sourcePath],
+    names: [],
+    mappings: 'AAAA',
+  };
+}