diff --git a/packages/connect-multichain/CHANGELOG.md b/packages/connect-multichain/CHANGELOG.md
index 1540a598..abc35294 100644
--- a/packages/connect-multichain/CHANGELOG.md
+++ b/packages/connect-multichain/CHANGELOG.md
@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+
+- Fix `PrivateKey is not a constructor` when connecting over the MWP transport from a bundled browser app. `eciesjs` publishes CommonJS only — no `module` field, and no `import` condition in its `exports` map — so bundlers shape `await import('eciesjs')` as `{ default: <module.exports> }` rather than a namespace carrying the named exports, leaving `decrypt`, `encrypt`, `PrivateKey` and `PublicKey` `undefined`. `createKeyManager` now unwraps `default` when it is present. Node and dev servers that synthesize the named exports were unaffected. ([#343](https://github.com/MetaMask/connect-monorepo/pull/343))
+
 ## [1.2.0]
 
 ### Added
diff --git a/packages/connect-multichain/src/multichain/transports/mwp/KeyManager.test.ts b/packages/connect-multichain/src/multichain/transports/mwp/KeyManager.test.ts
new file mode 100644
index 00000000..4bc1626c
--- /dev/null
+++ b/packages/connect-multichain/src/multichain/transports/mwp/KeyManager.test.ts
@@ -0,0 +1,47 @@
+/* eslint-disable id-length -- vitest alias */
+import type { IKeyManager } from '@metamask/mobile-wallet-protocol-core';
+import * as t from 'vitest';
+import { vi } from 'vitest';
+
+import { createKeyManager } from './KeyManager';
+
+const PLAINTEXT = 'hello from the dapp';
+
+/**
+ * Exercises every method of the key manager against real `eciesjs`.
+ *
+ * @param keymanager - The key manager under test.
+ * @returns The decrypted plaintext, which should match what was encrypted.
+ */
+async function roundTrip(keymanager: IKeyManager): Promise<string> {
+  const { privateKey, publicKey } = keymanager.generateKeyPair();
+  keymanager.validatePeerKey(publicKey);
+  const ciphertext = await keymanager.encrypt(PLAINTEXT, publicKey);
+  return keymanager.decrypt(ciphertext, privateKey);
+}
+
+t.describe('createKeyManager', () => {
+  t.it('round-trips with the namespace shape Node resolves', async () => {
+    const keymanager = await createKeyManager();
+
+    t.expect(await roundTrip(keymanager)).toBe(PLAINTEXT);
+  });
+
+  t.it('round-trips when the import resolves to `{ default }`', async () => {
+    // The shape bundlers produce for `eciesjs`, which publishes CommonJS only.
+    // Destructuring it directly yields `undefined` for every named export.
+    const actual = await vi.importActual('eciesjs');
+    vi.resetModules();
+    vi.doMock('eciesjs', () => ({ default: actual }));
+
+    try {
+      const { createKeyManager: create } = await import('./KeyManager');
+      const keymanager = await create();
+
+      t.expect(await roundTrip(keymanager)).toBe(PLAINTEXT);
+    } finally {
+      vi.doUnmock('eciesjs');
+      vi.resetModules();
+    }
+  });
+});
diff --git a/packages/connect-multichain/src/multichain/transports/mwp/KeyManager.ts b/packages/connect-multichain/src/multichain/transports/mwp/KeyManager.ts
index 4d3862b6..2f87a85b 100644
--- a/packages/connect-multichain/src/multichain/transports/mwp/KeyManager.ts
+++ b/packages/connect-multichain/src/multichain/transports/mwp/KeyManager.ts
@@ -16,7 +16,10 @@ import type {
  * @returns A ready-to-use key manager instance.
  */
 export async function createKeyManager(): Promise<IKeyManager> {
-  const { decrypt, encrypt, PrivateKey, PublicKey } = await import('eciesjs');
+  // `eciesjs` is CommonJS-only, so bundlers shape this import as
+  // `{ default: <module.exports> }` rather than a namespace of named exports.
+  const ecies = await import('eciesjs');
+  const { decrypt, encrypt, PrivateKey, PublicKey } = ecies.default ?? ecies;
 
   return {
     generateKeyPair(): KeyPair {