diff --git a/Doc/whatsnew/3.5.rst b/Doc/whatsnew/3.5.rst
index 0abdc3ff1ce..10a9a2b32f0 100644
--- a/Doc/whatsnew/3.5.rst
+++ b/Doc/whatsnew/3.5.rst
@@ -97,6 +97,10 @@ Implementation improvements:
 
 Significantly Improved Library Modules:
 
+* :class:`collections.OrderedDict` is now implemented in C, which improves
+  its performance between 4x to 100x times.  Contributed by Eric Snow in
+  :issue:`16991`.
+
 * You may now pass bytes to the :mod:`tempfile` module's APIs and it will
   return the temporary pathname as bytes instead of str.  It also accepts
   a value of ``None`` on parameters where only str was accepted in the past to