diff --git a/lib/internal/modules/package_map.js b/lib/internal/modules/package_map.js
index 38e7e12206c9..f5468390cd91 100644
--- a/lib/internal/modules/package_map.js
+++ b/lib/internal/modules/package_map.js
@@ -4,8 +4,10 @@ const {
   JSONParse,
   ObjectEntries,
   SafeMap,
+  StringPrototypeEndsWith,
   StringPrototypeIndexOf,
   StringPrototypeSlice,
+  StringPrototypeStartsWith,
 } = primordials;
 
 const assert = require('internal/assert');
@@ -19,6 +21,7 @@ const getPackageMapPath = getLazy(() =>
 const fs = require('fs');
 
 const {
+  ERR_INVALID_MODULE_SPECIFIER,
   ERR_PACKAGE_MAP_EXTERNAL_FILE,
   ERR_PACKAGE_MAP_INVALID,
   ERR_PACKAGE_MAP_KEY_NOT_FOUND,
@@ -193,6 +196,18 @@ class PackageMap {
       throw new ERR_PACKAGE_MAP_KEY_NOT_FOUND(targetKey, this.#configPath);
     }
 
+    if (subpath !== '.') {
+      let packageBaseURL = pathToFileURL(targetEntry.path);
+      if (!StringPrototypeEndsWith(packageBaseURL.pathname, '/')) {
+        packageBaseURL = new URL(`${packageBaseURL.href}/`);
+      }
+      const resolved = new URL(subpath, packageBaseURL);
+
+      if (!StringPrototypeStartsWith(resolved.pathname, packageBaseURL.pathname)) {
+        throw new ERR_INVALID_MODULE_SPECIFIER(specifier, 'resolves outside its package', parentPath);
+      }
+    }
+
     return { packagePath: targetEntry.path, subpath };
   }
 }
diff --git a/test/fixtures/package-map/traversal/app/index.mjs b/test/fixtures/package-map/traversal/app/index.mjs
new file mode 100644
index 000000000000..5519147ba5af
--- /dev/null
+++ b/test/fixtures/package-map/traversal/app/index.mjs
@@ -0,0 +1 @@
+export default 'app';
diff --git a/test/fixtures/package-map/traversal/dep/sub/file.cjs b/test/fixtures/package-map/traversal/dep/sub/file.cjs
new file mode 100644
index 000000000000..6aa967e0f0ae
--- /dev/null
+++ b/test/fixtures/package-map/traversal/dep/sub/file.cjs
@@ -0,0 +1 @@
+module.exports = 'dep-sub-cjs';
diff --git a/test/fixtures/package-map/traversal/dep/sub/file.mjs b/test/fixtures/package-map/traversal/dep/sub/file.mjs
new file mode 100644
index 000000000000..d41f326f0899
--- /dev/null
+++ b/test/fixtures/package-map/traversal/dep/sub/file.mjs
@@ -0,0 +1 @@
+export default 'dep-sub';
diff --git a/test/fixtures/package-map/traversal/map.json b/test/fixtures/package-map/traversal/map.json
new file mode 100644
index 000000000000..0220f099cf40
--- /dev/null
+++ b/test/fixtures/package-map/traversal/map.json
@@ -0,0 +1,6 @@
+{
+  "packages": {
+    "app": { "url": "./app", "dependencies": { "dep": "dep" } },
+    "dep": { "url": "./dep", "dependencies": {} }
+  }
+}
diff --git a/test/fixtures/package-map/traversal/secret.cjs b/test/fixtures/package-map/traversal/secret.cjs
new file mode 100644
index 000000000000..49563583c939
--- /dev/null
+++ b/test/fixtures/package-map/traversal/secret.cjs
@@ -0,0 +1 @@
+module.exports = 'SECRET-OUTSIDE-CJS';
diff --git a/test/fixtures/package-map/traversal/secret.mjs b/test/fixtures/package-map/traversal/secret.mjs
new file mode 100644
index 000000000000..a9c45fdb3b79
--- /dev/null
+++ b/test/fixtures/package-map/traversal/secret.mjs
@@ -0,0 +1 @@
+export default 'SECRET-OUTSIDE';
diff --git a/test/parallel/test-package-map-traversal.mjs b/test/parallel/test-package-map-traversal.mjs
new file mode 100644
index 000000000000..d63a3f120d92
--- /dev/null
+++ b/test/parallel/test-package-map-traversal.mjs
@@ -0,0 +1,69 @@
+import '../common/index.mjs';
+import { spawnSyncAndAssert, spawnSyncAndExit } from '../common/child_process.js';
+import * as fixtures from '../common/fixtures.mjs';
+
+const map = fixtures.path('package-map/traversal/map.json');
+const cwd = fixtures.path('package-map/traversal/app');
+
+function runESM(specifier) {
+  return [
+    '--no-warnings',
+    '--experimental-package-map', map,
+    '--input-type=module',
+    '--eval', `import x from '${specifier}'; console.log(x);`,
+  ];
+}
+
+function runCJS(specifier) {
+  return [
+    '--no-warnings',
+    '--experimental-package-map', map,
+    '--eval', `console.log(require('${specifier}'));`,
+  ];
+}
+
+for (const specifier of [
+  'dep/../secret.mjs',
+  'dep/%2e%2e/secret.mjs',
+  'dep/sub/../../secret.mjs',
+]) {
+  spawnSyncAndExit(process.execPath, runESM(specifier), { cwd }, {
+    status: 1,
+    signal: null,
+    stderr: /ERR_INVALID_MODULE_SPECIFIER/,
+  });
+}
+
+for (const specifier of [
+  'dep/sub/file.mjs',
+  'dep/./sub/file.mjs',
+  'dep/sub/../sub/file.mjs',
+]) {
+  spawnSyncAndAssert(process.execPath, runESM(specifier), { cwd }, {
+    stdout: /dep-sub/,
+    trim: true,
+  });
+}
+
+for (const specifier of [
+  'dep/../secret.cjs',
+  'dep/%2e%2e/secret.cjs',
+  'dep/sub/../../secret.cjs',
+]) {
+  spawnSyncAndExit(process.execPath, runCJS(specifier), { cwd }, {
+    status: 1,
+    signal: null,
+    stderr: /ERR_INVALID_MODULE_SPECIFIER/,
+  });
+}
+
+for (const specifier of [
+  'dep/sub/file.cjs',
+  'dep/./sub/file.cjs',
+  'dep/sub/../sub/file.cjs',
+]) {
+  spawnSyncAndAssert(process.execPath, runCJS(specifier), { cwd }, {
+    stdout: /dep-sub-cjs/,
+    trim: true,
+  });
+}