diff --git a/activesupport/lib/active_support/duration.rb b/activesupport/lib/active_support/duration.rb
index db5afb5324dc3..cd0d66a4825e6 100644
--- a/activesupport/lib/active_support/duration.rb
+++ b/activesupport/lib/active_support/duration.rb
@@ -38,6 +38,7 @@ def -@ #:nodoc:
     def is_a?(klass) #:nodoc:
       Duration == klass || value.is_a?(klass)
     end
+    alias :kind_of? :is_a?
 
     # Returns true if <tt>other</tt> is also a Duration instance with the
     # same <tt>value</tt>, or if <tt>other == value</tt>.
diff --git a/activesupport/test/core_ext/duration_test.rb b/activesupport/test/core_ext/duration_test.rb
index 05f529dc7dcc0..710c221fac296 100644
--- a/activesupport/test/core_ext/duration_test.rb
+++ b/activesupport/test/core_ext/duration_test.rb
@@ -5,6 +5,7 @@ class DurationTest < ActiveSupport::TestCase
   def test_is_a
     d = 1.day
     assert d.is_a?(ActiveSupport::Duration)
+    assert_kind_of ActiveSupport::Duration, d
     assert_kind_of Numeric, d
     assert_kind_of Fixnum, d
     assert !d.is_a?(Hash)