diff --git a/activesupport/CHANGELOG.md b/activesupport/CHANGELOG.md
index 41fde553fb1..fb975dcfc3d 100644
--- a/activesupport/CHANGELOG.md
+++ b/activesupport/CHANGELOG.md
@@ -1,5 +1,7 @@
 ## Rails 4.0.0 (unreleased) ##
 
+*   Introduce assert_not to replace warty 'assert !foo'.  *Jeremy Kemper*
+
 *   Prevent `Callbacks#set_callback` from setting the same callback twice.
 
         before_save :foo, :bar, :foo
diff --git a/activesupport/lib/active_support/testing/assertions.rb b/activesupport/lib/active_support/testing/assertions.rb
index 8466049e201..88aebba5c5b 100644
--- a/activesupport/lib/active_support/testing/assertions.rb
+++ b/activesupport/lib/active_support/testing/assertions.rb
@@ -3,6 +3,22 @@ require 'active_support/core_ext/object/blank'
 module ActiveSupport
   module Testing
     module Assertions
+      # Assert that an expression is not truthy. Passes if <tt>object</tt> is
+      # +nil+ or +false+. "Truthy" means "considered true in a conditional"
+      # like <tt>if foo</tt>.
+      #
+      #   assert_not nil    # => true
+      #   assert_not false  # => true
+      #   assert_not 'foo'  # => 'foo' is not nil or false
+      #
+      # An error message can be specified.
+      #
+      #   assert_not foo, 'foo should be false'
+      def assert_not(object, message = nil)
+        message ||= "Expected #{mu_pp(object)} to be nil or false"
+        assert !object, message
+      end
+
       # Test numeric difference between the return value of an expression as a
       # result of what is evaluated in the yielded block.
       #
diff --git a/activesupport/test/test_test.rb b/activesupport/test/test_test.rb
index 7f732a1d20b..44fe6487104 100644
--- a/activesupport/test/test_test.rb
+++ b/activesupport/test/test_test.rb
@@ -15,6 +15,27 @@ class AssertDifferenceTest < ActiveSupport::TestCase
     @object.num = 0
   end
 
+  def test_assert_not
+    assert_not nil
+    assert_not false
+
+    begin
+      assert_not true
+    rescue Exception => e
+      assert_equal 'Expected true to be nil or false', e.message
+    else
+      fail 'assert_not true should fail'
+    end
+
+    begin
+      assert_not true, 'custom'
+    rescue Exception => e
+      assert_equal 'custom', e.message
+    else
+      fail 'assert_not true should fail'
+    end
+  end
+
   def test_assert_no_difference
     assert_no_difference '@object.num' do
       # ...