diff --git a/django/middleware/sessions.py b/django/middleware/sessions.py
index df7473e904..545ee38fa8 100644
--- a/django/middleware/sessions.py
+++ b/django/middleware/sessions.py
@@ -11,6 +11,9 @@ class SessionWrapper(object):
         self.session_key = session_key
         self.modified = False
 
+    def __contains__(self, key):
+        return key in self._session
+
     def __getitem__(self, key):
         return self._session[key]
 
diff --git a/docs/sessions.txt b/docs/sessions.txt
index bea246e17d..fbb11e4198 100644
--- a/docs/sessions.txt
+++ b/docs/sessions.txt
@@ -34,6 +34,9 @@ it and write to it.
 
 It implements the following standard dictionary methods:
 
+    * ``__contains__(key)``
+      **New in Django development version.** Example: ``'fav_color' in request.session``
+
     * ``__getitem__(key)``
       Example: ``fav_color = request.session['fav_color']``