diff --git a/actionpack/CHANGELOG b/actionpack/CHANGELOG
index a3b0d6cfb7..7f4256d603 100644
--- a/actionpack/CHANGELOG
+++ b/actionpack/CHANGELOG
@@ -1,5 +1,7 @@
 *SVN*
 
+* Fix bug with setting Request#format= after the getter has cached the value.  #10889 [cch1]
+
 * Correct inconsistencies in RequestForgeryProtection docs.  #11032 [mislav]
 
 * Introduce a Template class to ActionView.  #11024 [lifofifo]
diff --git a/actionpack/lib/action_controller/request.rb b/actionpack/lib/action_controller/request.rb
index e3892037d8..df8dbdc789 100755
--- a/actionpack/lib/action_controller/request.rb
+++ b/actionpack/lib/action_controller/request.rb
@@ -113,7 +113,7 @@ module ActionController
     #   end
     def format=(extension)
       parameters[:format] = extension.to_s
-      format
+      @format = Mime::Type.lookup_by_extension(parameters[:format])
     end
 
     # Returns true if the request's "X-Requested-With" header contains
diff --git a/actionpack/test/controller/request_test.rb b/actionpack/test/controller/request_test.rb
index 520358f739..68d6afb426 100644
--- a/actionpack/test/controller/request_test.rb
+++ b/actionpack/test/controller/request_test.rb
@@ -371,6 +371,13 @@ class RequestTest < Test::Unit::TestCase
     assert_equal Mime::HTML, @request.content_type
   end
 
+  def test_format_assignment_should_set_format
+    @request.instance_eval { self.format = :txt }
+    assert !@request.format.xml?
+    @request.instance_eval { self.format = :xml }
+    assert @request.format.xml?
+  end
+
   def test_content_no_type
     assert_equal nil, @request.content_type
   end