diff --git a/NEWS b/NEWS
index 7ad0da64df26f014416a7bfac4e64f5b431c51d0..6b70b69e2304a321cb5bd4d1c348f88e7e6da59a 100644
--- a/NEWS
+++ b/NEWS
@@ -1,3 +1,5 @@
+- Issue #15323: improve failure message of Mock.assert_called_once_with
+
 - Issue #14857: fix regression in references to PEP 3135 implicit __class__
   closure variable (Reopens issue #12370)
 
diff --git a/mock.py b/mock.py
index e47f3b142fcb5a64e5e40e3e629daa9fcb7abdc5..39b0dd22ecaf4b89cbee294276d0f8f804afac1b 100644
--- a/mock.py
+++ b/mock.py
@@ -864,8 +864,8 @@ class NonCallableMock(Base):
         arguments."""
         self = _mock_self
         if not self.call_count == 1:
-            msg = ("Expected to be called once. Called %s times." %
-                   self.call_count)
+            msg = ("Expected '%s' to be called once. Called %s times." %
+                   (self._mock_name or 'mock', self.call_count))
             raise AssertionError(msg)
         return self.assert_called_with(*args, **kwargs)
 
diff --git a/tests/testmock.py b/tests/testmock.py
index b933e90f2840719f93378a19e90996064eb71f6e..d788ba0c2d4872fdf92a34b95ad5568982fa4f8d 100644
--- a/tests/testmock.py
+++ b/tests/testmock.py
@@ -486,6 +486,13 @@ class MockTest(unittest.TestCase):
                                 mock.assert_called_with)
 
 
+    def test_assert_called_once_with_message(self):
+        mock = Mock(name='geoffrey')
+        self.assertRaisesRegex(AssertionError,
+                     r"Expected 'geoffrey' to be called once\.",
+                     mock.assert_called_once_with)
+
+
     def test__name__(self):
         mock = Mock()
         self.assertRaises(AttributeError, lambda: mock.__name__)