@@ -20,6 +20,7 @@ const {
   ReflectConstruct,
   ReflectGet,
   SafeMap,
+  StringPrototypeIncludes,
   StringPrototypeSlice,
   StringPrototypeStartsWith,
 } = primordials;
@@ -207,6 +208,7 @@ class MockModuleContext {
     baseURL,
     cache,
     caller,
+    cjsPath,
     format,
     fullPath,
     moduleExports,
@@ -222,12 +224,25 @@ class MockModuleContext {
 
     sharedState.mockMap.set(baseURL, config);
     sharedState.mockMap.set(fullPath, config);
+    // For dual packages (e.g., a package with a "exports" field that exposes
+    // both ESM and CJS entry points), the file selected by the ESM resolver
+    // (used to compute fullPath) may differ from the one selected by CJS
+    // require(). Register the CJS-resolved path so that require() also picks
+    // up the mock. See https://github.com/nodejs/node/issues/58231.
+    if (cjsPath !== null && cjsPath !== fullPath) {
+      sharedState.mockMap.set(cjsPath, config);
+    }
 
     this.#sharedState = sharedState;
     this.#restore = {
       __proto__: null,
       baseURL,
       cached: fullPath in Module._cache,
+      cjsPath,
+      cjsCached: cjsPath !== null && cjsPath !== fullPath &&
+        cjsPath in Module._cache,
+      cjsValue: cjsPath !== null && cjsPath !== fullPath ?
+        Module._cache[cjsPath] : undefined,
       format,
       fullPath,
       value: Module._cache[fullPath],
@@ -257,6 +272,9 @@ class MockModuleContext {
     }
 
     delete Module._cache[fullPath];
+    if (cjsPath !== null && cjsPath !== fullPath) {
+      delete Module._cache[cjsPath];
+    }
     sharedState.mockExports.set(baseURL, {
       __proto__: null,
       moduleExports,
@@ -276,6 +294,14 @@ class MockModuleContext {
       Module._cache[this.#restore.fullPath] = this.#restore.value;
     }
 
+    if (this.#restore.cjsPath !== null &&
+        this.#restore.cjsPath !== this.#restore.fullPath) {
+      delete Module._cache[this.#restore.cjsPath];
+      if (this.#restore.cjsCached) {
+        Module._cache[this.#restore.cjsPath] = this.#restore.cjsValue;
+      }
+    }
+
     const mock = mocks.get(this.#restore.baseURL);
 
     if (mock !== undefined) {
@@ -285,6 +311,10 @@ class MockModuleContext {
 
     this.#sharedState.mockMap.delete(this.#restore.baseURL);
     this.#sharedState.mockMap.delete(this.#restore.fullPath);
+    if (this.#restore.cjsPath !== null &&
+        this.#restore.cjsPath !== this.#restore.fullPath) {
+      this.#sharedState.mockMap.delete(this.#restore.cjsPath);
+    }
     this.#restore = undefined;
   }
 }
@@ -680,11 +710,19 @@ class MockTracker {
 
     const fullPath = StringPrototypeStartsWith(url, 'file://') ?
       fileURLToPath(url) : null;
+    // For dual packages, the ESM resolver may return a different file than
+    // CJS require() would for the same specifier (e.g., when a package's
+    // "exports" field points to different files for the "import" and
+    // "require" conditions). Compute the CJS-resolved path so that
+    // require() of a mocked module also picks up the mock.
+    // See https://github.com/nodejs/node/issues/58231.
+    const cjsPath = resolveAsCJS(mockSpecifier, caller, fullPath);
     const ctx = new MockModuleContext({
       __proto__: null,
       baseURL: baseURL.href,
       cache,
       caller,
+      cjsPath,
       format,
       fullPath,
       moduleExports,
@@ -987,6 +1025,54 @@ function cjsMockModuleLoad(request, parent, isMain) {
   return modExports;
 }
 
+// Resolve `specifier` using CJS resolution rules so that mocks for dual
+// packages (e.g., a package whose "exports" field points to different files
+// for the "import" and "require" conditions) also intercept require().
+// Returns an absolute file path on success, or null when the specifier cannot
+// be resolved as CJS (for example, when the package is ESM-only or when it is
+// a non-file URL such as data: or node:).
+function resolveAsCJS(specifier, callerURL, esmFullPath) {
+  if (isBuiltin(specifier) ||
+      StringPrototypeStartsWith(specifier, 'node:') ||
+      StringPrototypeStartsWith(specifier, 'data:')) {
+    return null;
+  }
+
+  let parentPath;
+  if (StringPrototypeStartsWith(callerURL, 'file://')) {
+    try {
+      parentPath = fileURLToPath(callerURL);
+    } catch {
+      return null;
+    }
+  } else {
+    return null;
+  }
+
+  try {
+    const tmpModule = new Module(parentPath, null);
+    tmpModule.paths = _nodeModulePaths(parentPath);
+    const resolved = _resolveFilename(specifier, tmpModule, false);
+    if (typeof resolved !== 'string') {
+      return null;
+    }
+    // If the resolution matches what the ESM resolver picked, there is
+    // nothing additional to register.
+    if (resolved === esmFullPath) {
+      return esmFullPath;
+    }
+    // If the resolution returned something that is not a filesystem path
+    // (e.g., a builtin id without a slash or backslash), ignore it.
+    if (!StringPrototypeIncludes(resolved, '/') &&
+        !StringPrototypeIncludes(resolved, '\\')) {
+      return null;
+    }
+    return resolved;
+  } catch {
+    return null;
+  }
+}
+
 function validateStringOrSymbol(value, name) {
   if (typeof value !== 'string' && typeof value !== 'symbol') {
     throw new ERR_INVALID_ARG_TYPE(name, ['string', 'symbol'], value);
 
@@ -0,0 +1,32 @@
+'use strict';
+const assert = require('node:assert');
+const { test } = require('node:test');
+const fixture = 'dual-pkg-with-exports';
+
+test('mock node_modules dual package with conditional exports', async (t) => {
+  const mock = t.mock.module(fixture, {
+    namedExports: { add(x, y) { return 1 + x + y; }, flavor: 'mocked' },
+  });
+
+  // CJS require should pick up the mock even though the package's "exports"
+  // field maps the "require" condition to a different file than "import".
+  const cjsImpl = require(fixture);
+  assert.strictEqual(cjsImpl.add(4, 5), 10);
+  assert.strictEqual(cjsImpl.flavor, 'mocked');
+
+  // ESM dynamic import should also pick up the mock.
+  const esmImpl = await import(fixture);
+  assert.strictEqual(esmImpl.add(4, 5), 10);
+  assert.strictEqual(esmImpl.flavor, 'mocked');
+
+  mock.restore();
+
+  // After restore, both module systems should see the original exports.
+  const restoredCjs = require(fixture);
+  assert.strictEqual(restoredCjs.add(4, 5), 9);
+  assert.strictEqual(restoredCjs.flavor, 'cjs');
+
+  const restoredEsm = await import(fixture);
+  assert.strictEqual(restoredEsm.add(4, 5), 9);
+  assert.strictEqual(restoredEsm.flavor, 'esm');
+});
@@ -0,0 +1,36 @@
+'use strict';
+const common = require('../common');
+const { isMainThread } = require('worker_threads');
+
+if (!isMainThread) {
+  common.skip('registering customization hooks in Workers does not work');
+}
+
+const fixtures = require('../common/fixtures');
+const assert = require('node:assert');
+const { test } = require('node:test');
+
+// Regression test for https://github.com/nodejs/node/issues/58231
+// When a dual package exposes both ESM and CJS entry points via the
+// "exports" field with "import"/"require" conditions, the ESM resolver
+// picks one file (e.g. index.js) and CJS require() picks another
+// (e.g. index.cjs). mock.module() must intercept both so that require()
+// of the mocked module does not return the original CJS file.
+test('mock.module intercepts dual package require with conditional exports',
+     async () => {
+       const cwd = fixtures.path('test-runner');
+       const fixture = fixtures.path('test-runner', 'mock-nm-dual-pkg.js');
+       const args = ['--experimental-test-module-mocks', fixture];
+       const {
+         code,
+         stdout,
+         signal,
+       } = await common.spawnPromisified(process.execPath, args, { cwd });
+
+       assert.strictEqual(signal, null);
+       assert.strictEqual(code, 0,
+                          'child process exited with non-zero status\n' +
+                          `stdout:\n${stdout}`);
+       assert.match(stdout, /pass 1/);
+       assert.match(stdout, /fail 0/);
+     });