diff --git a/.changeset/private-key-jwt-eddsa-pem.md b/.changeset/private-key-jwt-eddsa-pem.md
new file mode 100644
index 0000000000..39ca95df2a
--- /dev/null
+++ b/.changeset/private-key-jwt-eddsa-pem.md
@@ -0,0 +1,5 @@
+---
+'@modelcontextprotocol/client': patch
+---
+
+`PrivateKeyJwtProvider` and `createPrivateKeyJwtAuth()` now sign with an Ed25519 key given as a PEM string when the algorithm is `EdDSA` or `Ed25519`. They threw `Unsupported algorithm EdDSA` before, although the same key given as a `Uint8Array` or a JWK already worked.
diff --git a/packages/client/src/client/authExtensions.ts b/packages/client/src/client/authExtensions.ts
index e210786b7e..c7d6477408 100644
--- a/packages/client/src/client/authExtensions.ts
+++ b/packages/client/src/client/authExtensions.ts
@@ -63,7 +63,7 @@ export function createPrivateKeyJwtAuth(options: {
         const alg = options.alg;
         let key: unknown;
         if (typeof options.privateKey === 'string') {
-            if (alg.startsWith('RS') || alg.startsWith('ES') || alg.startsWith('PS')) {
+            if (alg.startsWith('RS') || alg.startsWith('ES') || alg.startsWith('PS') || alg.startsWith('Ed')) {
                 key = await jose.importPKCS8(options.privateKey, alg);
             } else if (alg.startsWith('HS')) {
                 key = new TextEncoder().encode(options.privateKey);
diff --git a/packages/client/test/client/authExtensions.test.ts b/packages/client/test/client/authExtensions.test.ts
index cb4f36a1e1..cdfdcdd960 100644
--- a/packages/client/test/client/authExtensions.test.ts
+++ b/packages/client/test/client/authExtensions.test.ts
@@ -393,6 +393,26 @@ describe('createPrivateKeyJwtAuth', () => {
         expect(parts).toHaveLength(3);
     });
 
+    it.each(['EdDSA', 'Ed25519'])('creates a verifiable JWT when using an Ed25519 PEM private key with alg %s', async alg => {
+        const jose = await import('jose');
+        const { privateKey, publicKey } = await jose.generateKeyPair(alg, { extractable: true });
+        const pem = await jose.exportPKCS8(privateKey);
+
+        const addClientAuth = createPrivateKeyJwtAuth({
+            issuer: 'client-id',
+            subject: 'client-id',
+            privateKey: pem,
+            alg
+        });
+
+        const params = new URLSearchParams();
+        await addClientAuth(new Headers(), params, 'https://auth.example.com/token', undefined);
+
+        const { payload, protectedHeader } = await jose.jwtVerify(params.get('client_assertion')!, publicKey);
+        expect(protectedHeader.alg).toBe(alg);
+        expect(payload.iss).toBe('client-id');
+    });
+
     it('uses metadata.issuer as audience when available', async () => {
         const addClientAuth = createPrivateKeyJwtAuth(baseOptions);