diff --git a/lib/SessionManager.php b/lib/SessionManager.php
index 131d67e3..2855632b 100644
--- a/lib/SessionManager.php
+++ b/lib/SessionManager.php
@@ -132,6 +132,8 @@ public static function sealSessionFromAuthResponse(
      * @param string $cookiePassword The encryption key.
      * @param string $clientId The WorkOS client ID (for JWKS URL).
      * @param string $baseUrl The WorkOS API base URL. Defaults to 'https://api.workos.com/'.
+     * @param string|array<string>|null $issuer Expected `iss` claim (one issuer or a list of
+     *   accepted issuers). When null, the issuer is not validated.
      * @return array Authentication result.
      */
     public function authenticate(
@@ -139,6 +141,7 @@ public function authenticate(
         string $cookiePassword,
         string $clientId,
         string $baseUrl = 'https://api.workos.com/',
+        string|array|null $issuer = null,
     ): array {
         if (empty($sessionData)) {
             return [
@@ -164,7 +167,7 @@ public function authenticate(
         }
 
         try {
-            $decoded = $this->decodeAccessToken($session['access_token'], $clientId);
+            $decoded = $this->decodeAccessToken($session['access_token'], $clientId, $issuer);
         } catch (\Exception $e) {
             return [
                 'authenticated' => false,
@@ -263,6 +266,7 @@ public function refresh(
      * @param string $clientId The WorkOS client ID.
      * @param string|null $returnTo Optional URL to redirect to after logout.
      * @param string $baseUrl The WorkOS API base URL.
+     * @param string|array<string>|null $issuer Expected `iss` claim; see {@see authenticate()}.
      * @return string The logout URL.
      * @throws \InvalidArgumentException If the session cannot be authenticated.
      */
@@ -272,8 +276,9 @@ public function getLogoutUrl(
         string $clientId,
         ?string $returnTo = null,
         string $baseUrl = 'https://api.workos.com/',
+        string|array|null $issuer = null,
     ): string {
-        $authResult = $this->authenticate($sessionData, $cookiePassword, $clientId, $baseUrl);
+        $authResult = $this->authenticate($sessionData, $cookiePassword, $clientId, $baseUrl, $issuer);
 
         if (!$authResult['authenticated']) {
             throw new \InvalidArgumentException(
@@ -361,17 +366,21 @@ private function getCachedJwks(string $clientId, bool $forceRefresh = false): ar
      * Decode and validate an access token JWT.
      *
      * Verifies the JWS signature against the JWKS published for `$clientId`,
-     * enforces an algorithm allow-list, and rejects expired tokens. This is
-     * the only path used by {@see authenticate()}; callers must not bypass it.
+     * enforces an algorithm allow-list, rejects expired tokens, and — when
+     * `$issuer` is given — requires the `iss` claim to match one of the
+     * accepted issuers. This is the only path used by {@see authenticate()};
+     * callers must not bypass it.
      *
      * @param string $accessToken The JWT access token.
      * @param string $clientId The WorkOS client ID (used to fetch JWKS).
+     * @param string|array<string>|null $issuer Accepted `iss` value(s), or null to skip the check.
      * @return array The decoded JWT claims.
      * @throws \InvalidArgumentException If the token cannot be decoded or fails verification.
      */
     private function decodeAccessToken(
         string $accessToken,
         string $clientId,
+        string|array|null $issuer = null,
     ): array {
         $parts = explode('.', $accessToken);
         if (count($parts) !== 3) {
@@ -441,11 +450,20 @@ private function decodeAccessToken(
             throw new \InvalidArgumentException('JWT has expired');
         }
 
-        // TODO(security-fix-plan.md, finding #60): enforce documented WorkOS
-        // `iss` and `aud` values once empirically confirmed. The other WorkOS
-        // SDKs (Ruby, Python) currently skip `aud` verification, so the
-        // canonical values are not authoritatively documented in this repo.
-        // Track resolution under "Open questions / follow-ups" in the plan.
+        if ($issuer !== null) {
+            $accepted = is_array($issuer) ? $issuer : [$issuer];
+            $iss = $decoded['iss'] ?? null;
+            if (!is_string($iss) || !in_array($iss, $accepted, true)) {
+                throw new \InvalidArgumentException('JWT issuer mismatch');
+            }
+        }
+
+        // TODO(security-fix-plan.md, finding #60): enforce `iss` and `aud` by
+        // default once the canonical WorkOS values are empirically confirmed.
+        // The other WorkOS SDKs (Ruby, Python) currently skip `aud` verification
+        // and only check `iss` when configured, so the canonical values are not
+        // authoritatively documented in this repo. Track resolution under
+        // "Open questions / follow-ups" in the plan.
 
         return $decoded;
     }
diff --git a/tests/SessionManagerTest.php b/tests/SessionManagerTest.php
index 36fa1c7d..9ee86b93 100644
--- a/tests/SessionManagerTest.php
+++ b/tests/SessionManagerTest.php
@@ -203,6 +203,77 @@ public function testAuthenticateValidatesSignedJwt(): void
         $this->assertSame('org_test', $result['organization_id']);
     }
 
+    /**
+     * @param string|array<string>|null $issuer
+     * @return array<string, mixed>
+     */
+    private function authenticateWithIssuer(string|array|null $issuer, ?string $iss): array
+    {
+        $claims = ['sid' => 'session_iss', 'exp' => time() + 3600];
+        if ($iss !== null) {
+            $claims['iss'] = $iss;
+        }
+        [$jwks, $jwt] = $this->buildSignedJwt($claims);
+
+        $sealed = SessionManager::sealSessionFromAuthResponse(
+            accessToken: $jwt,
+            refreshToken: 'ref_test',
+            cookiePassword: $this->cookiePassword,
+        );
+
+        $client = $this->createMockClient([['status' => 200, 'body' => $jwks]]);
+
+        return $client->sessionManager()->authenticate(
+            sessionData: $sealed,
+            cookiePassword: $this->cookiePassword,
+            clientId: 'client_123',
+            issuer: $issuer,
+        );
+    }
+
+    public function testAuthenticateIgnoresIssuerWhenNotConfigured(): void
+    {
+        $result = $this->authenticateWithIssuer(null, 'https://other.example.com');
+        $this->assertTrue($result['authenticated']);
+    }
+
+    public function testAuthenticateAcceptsMatchingIssuer(): void
+    {
+        $result = $this->authenticateWithIssuer('https://api.workos.com', 'https://api.workos.com');
+        $this->assertTrue($result['authenticated']);
+        $this->assertSame('session_iss', $result['session_id']);
+    }
+
+    public function testAuthenticateRejectsMismatchedIssuer(): void
+    {
+        $result = $this->authenticateWithIssuer('https://api.workos.com', 'https://other.example.com');
+        $this->assertFalse($result['authenticated']);
+        $this->assertSame('invalid_jwt', $result['reason']);
+    }
+
+    public function testAuthenticateRejectsMissingIssWhenIssuerConfigured(): void
+    {
+        $result = $this->authenticateWithIssuer('https://api.workos.com', null);
+        $this->assertFalse($result['authenticated']);
+        $this->assertSame('invalid_jwt', $result['reason']);
+    }
+
+    public function testAuthenticateAcceptsAnyListedIssuer(): void
+    {
+        $result = $this->authenticateWithIssuer(
+            ['https://api.workos.com', 'https://auth.example.com'],
+            'https://auth.example.com',
+        );
+        $this->assertTrue($result['authenticated']);
+    }
+
+    public function testAuthenticateRejectsAllTokensWhenIssuerListIsEmpty(): void
+    {
+        $result = $this->authenticateWithIssuer([], 'https://api.workos.com');
+        $this->assertFalse($result['authenticated']);
+        $this->assertSame('invalid_jwt', $result['reason']);
+    }
+
     public function testAuthenticateRejectsTamperedSignature(): void
     {
         [$jwks, $jwt] = $this->buildSignedJwt([