diff --git a/google/auth/app_engine.py b/google/auth/app_engine.py
index 8b17e76..e6e84d2 100644
--- a/google/auth/app_engine.py
+++ b/google/auth/app_engine.py
@@ -123,6 +123,7 @@
         super(Credentials, self).__init__()
         self._scopes = scopes
         self._service_account_id = service_account_id
+        self._signer = Signer()
 
     @_helpers.copy_docstring(credentials.Credentials)
     def refresh(self, request):
@@ -156,9 +157,14 @@
 
     @_helpers.copy_docstring(credentials.Signing)
     def sign_bytes(self, message):
-        return Signer().sign(message)
+        return self._signer.sign(message)
 
     @property
     @_helpers.copy_docstring(credentials.Signing)
     def signer_email(self):
         return self.service_account_email
+
+    @property
+    @_helpers.copy_docstring(credentials.Signing)
+    def signer(self):
+        return self._signer
diff --git a/google/auth/credentials.py b/google/auth/credentials.py
index 360dc0e..2358b1d 100644
--- a/google/auth/credentials.py
+++ b/google/auth/credentials.py
@@ -236,3 +236,10 @@
         # pylint: disable=missing-raises-doc
         # (pylint doesn't recognize that this is abstract)
         raise NotImplementedError('Signer email must be implemented.')
+
+    @abc.abstractproperty
+    def signer(self):
+        """google.auth.crypt.Signer: The signer used to sign bytes."""
+        # pylint: disable=missing-raises-doc
+        # (pylint doesn't recognize that this is abstract)
+        raise NotImplementedError('Signer must be implemented.')
diff --git a/google/auth/jwt.py b/google/auth/jwt.py
index dfaf2e6..418be6b 100644
--- a/google/auth/jwt.py
+++ b/google/auth/jwt.py
@@ -468,6 +468,11 @@
     def signer_email(self):
         return self._issuer
 
+    @property
+    @_helpers.copy_docstring(credentials.Signing)
+    def signer(self):
+        return self._signer
+
     def before_request(self, request, method, url, headers):
         """Performs credential-specific before request logic.