diff --git a/google/auth/app_engine.py b/google/auth/app_engine.py
index 566475e..608651d 100644
--- a/google/auth/app_engine.py
+++ b/google/auth/app_engine.py
@@ -110,3 +110,8 @@
     @_helpers.copy_docstring(credentials.Signing)
     def sign_bytes(self, message):
         return app_identity.sign_blob(message)
+
+    @property
+    @_helpers.copy_docstring(credentials.Signing)
+    def signer_email(self):
+        return self.service_account_email
diff --git a/google/auth/credentials.py b/google/auth/credentials.py
index 470f1a4..360dc0e 100644
--- a/google/auth/credentials.py
+++ b/google/auth/credentials.py
@@ -229,3 +229,10 @@
         # pylint: disable=missing-raises-doc,redundant-returns-doc
         # (pylint doesn't recognize that this is abstract)
         raise NotImplementedError('Sign bytes must be implemented.')
+
+    @abc.abstractproperty
+    def signer_email(self):
+        """Optional[str]: An email address that identifies the signer."""
+        # pylint: disable=missing-raises-doc
+        # (pylint doesn't recognize that this is abstract)
+        raise NotImplementedError('Signer email must be implemented.')
diff --git a/google/auth/jwt.py b/google/auth/jwt.py
index 0884b3d..dfaf2e6 100644
--- a/google/auth/jwt.py
+++ b/google/auth/jwt.py
@@ -463,6 +463,11 @@
         """
         return self._signer.sign(message)
 
+    @property
+    @_helpers.copy_docstring(credentials.Signing)
+    def signer_email(self):
+        return self._issuer
+
     def before_request(self, request, method, url, headers):
         """Performs credential-specific before request logic.