diff --git a/google/auth/_helpers.py b/google/auth/_helpers.py
index 9c49e06..9fe2caa 100644
--- a/google/auth/_helpers.py
+++ b/google/auth/_helpers.py
@@ -21,6 +21,20 @@
 from six.moves import urllib
 
 
+def copy_docstring(source_class):
+    """Decorator that copies a method's docstring from another class."""
+    def decorator(method):
+        """Decorator implementation."""
+        if method.__doc__:
+            raise ValueError('Method already has a docstring.')
+
+        source_method = getattr(source_class, method.__name__)
+        method.__doc__ = source_method.__doc__
+
+        return method
+    return decorator
+
+
 def utcnow():
     """Returns the current UTC datetime.
 
diff --git a/tests/test__helpers.py b/tests/test__helpers.py
index 86cacef..35c16d4 100644
--- a/tests/test__helpers.py
+++ b/tests/test__helpers.py
@@ -20,6 +20,38 @@
 from google.auth import _helpers
 
 
+class SourceClass(object):
+    def func(self):  # pragma: NO COVER
+        """example docstring"""
+        pass
+
+
+def test_copy_docstring_success():
+    def func():  # pragma: NO COVER
+        pass
+
+    _helpers.copy_docstring(SourceClass)(func)
+
+    assert func.__doc__ == SourceClass.func.__doc__
+
+
+def test_copy_docstring_conflict():
+    def func():  # pragma: NO COVER
+        """existing docstring"""
+        pass
+
+    with pytest.raises(ValueError):
+        _helpers.copy_docstring(SourceClass)(func)
+
+
+def test_copy_docstring_non_existing():
+    def func2():  # pragma: NO COVER
+        pass
+
+    with pytest.raises(AttributeError):
+        _helpers.copy_docstring(SourceClass)(func2)
+
+
 def test_utcnow():
     assert isinstance(_helpers.utcnow(), datetime.datetime)