diff --git a/Doc/library/email.contentmanager.rst b/Doc/library/email.contentmanager.rst
index 8f0bfdbe531..8f33a146dd8 100644
--- a/Doc/library/email.contentmanager.rst
+++ b/Doc/library/email.contentmanager.rst
@@ -70,11 +70,15 @@ this module.
    the following methods:
 
 
-   .. attribute:: is_attachment
+   .. method:: is_attachment
 
-      Set to ``True`` if there is a :mailheader:`Content-Disposition` header
+      Return ``True`` if there is a :mailheader:`Content-Disposition` header
       and its (case insensitive) value is ``attachment``, ``False`` otherwise.
 
+      .. versionchanged:: 3.4.2
+         is_attachment is now a method instead of a property, for consistency
+         with :meth:`~email.message.Message.is_multipart`.
+
 
    .. method:: get_body(preferencelist=('related', 'html', 'plain'))
 
diff --git a/Lib/email/message.py b/Lib/email/message.py
index 124071d5e1d..a179f8e3742 100644
--- a/Lib/email/message.py
+++ b/Lib/email/message.py
@@ -9,6 +9,7 @@ __all__ = ['Message']
 import re
 import uu
 import quopri
+import warnings
 from io import BytesIO, StringIO
 
 # Intrapackage imports
@@ -929,6 +930,17 @@ class Message:
     # I.e. def walk(self): ...
     from email.iterators import walk
 
+# XXX Support for temporary deprecation hack for is_attachment property.
+class _IsAttachment:
+    def __init__(self, value):
+        self.value = value
+    def __call__(self):
+        return self.value
+    def __bool__(self):
+        warnings.warn("is_attachment will be a method, not a property, in 3.5",
+                      DeprecationWarning,
+                      stacklevel=3)
+        return self.value
 
 class MIMEPart(Message):
 
@@ -941,10 +953,12 @@ class MIMEPart(Message):
     @property
     def is_attachment(self):
         c_d = self.get('content-disposition')
-        return False if c_d is None else c_d.content_disposition == 'attachment'
+        result = False if c_d is None else c_d.content_disposition == 'attachment'
+        # XXX transitional hack to raise deprecation if not called.
+        return _IsAttachment(result)
 
     def _find_body(self, part, preferencelist):
-        if part.is_attachment:
+        if part.is_attachment():
             return
         maintype, subtype = part.get_content_type().split('/')
         if maintype == 'text':
@@ -1037,7 +1051,7 @@ class MIMEPart(Message):
         for part in parts:
             maintype, subtype = part.get_content_type().split('/')
             if ((maintype, subtype) in self._body_types and
-                    not part.is_attachment and subtype not in seen):
+                    not part.is_attachment() and subtype not in seen):
                 seen.append(subtype)
                 continue
             yield part
diff --git a/Lib/test/test_email/test_message.py b/Lib/test/test_email/test_message.py
index 51ddf360996..50e1a632c2b 100644
--- a/Lib/test/test_email/test_message.py
+++ b/Lib/test/test_email/test_message.py
@@ -722,15 +722,25 @@ class TestEmailMessageBase:
 
     def test_is_attachment(self):
         m = self._make_message()
-        self.assertFalse(m.is_attachment)
+        self.assertFalse(m.is_attachment())
+        with self.assertWarns(DeprecationWarning):
+            self.assertFalse(m.is_attachment)
         m['Content-Disposition'] = 'inline'
-        self.assertFalse(m.is_attachment)
+        self.assertFalse(m.is_attachment())
+        with self.assertWarns(DeprecationWarning):
+            self.assertFalse(m.is_attachment)
         m.replace_header('Content-Disposition', 'attachment')
-        self.assertTrue(m.is_attachment)
+        self.assertTrue(m.is_attachment())
+        with self.assertWarns(DeprecationWarning):
+            self.assertTrue(m.is_attachment)
         m.replace_header('Content-Disposition', 'AtTachMent')
-        self.assertTrue(m.is_attachment)
+        self.assertTrue(m.is_attachment())
+        with self.assertWarns(DeprecationWarning):
+            self.assertTrue(m.is_attachment)
         m.set_param('filename', 'abc.png', 'Content-Disposition')
-        self.assertTrue(m.is_attachment)
+        self.assertTrue(m.is_attachment())
+        with self.assertWarns(DeprecationWarning):
+            self.assertTrue(m.is_attachment)
 
 
 class TestEmailMessage(TestEmailMessageBase, TestEmailBase):
diff --git a/Misc/NEWS b/Misc/NEWS
index c5afa50c837..356021291a3 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -32,6 +32,11 @@ Core and Builtins
 Library
 -------
 
+- Issue #21091: Fix API bug: email.message.EmailMessage.is_attachment is now
+  a method.  Since EmailMessage is provisional, we can change the API in a
+  maintenance release, but we use a trick to remain backward compatible with
+  3.4.0/1.
+
 - Issue #21079: Fix email.message.EmailMessage.is_attachment to return the
   correct result when the header has parameters as well as a value.