diff --git a/activesupport/CHANGELOG.md b/activesupport/CHANGELOG.md
index 047688aba82a76ca69d914044e72264510c9f0df..20db6fb048eae3ddf6b1caed2acc533c64d16e6c 100644
--- a/activesupport/CHANGELOG.md
+++ b/activesupport/CHANGELOG.md
@@ -1,3 +1,31 @@
+*   Fix `thread_mattr_accessor` subclass no longer overwrites parent.
+
+    Assigning a value to a subclass using `thread_mattr_accessor` no
+    longer changes the value of the parent class. This brings the
+    behavior inline with the documentation.
+
+    Given:
+
+        class Account
+          thread_mattr_accessor :user
+        end
+
+        class Customer < Account
+        end
+
+        Account.user = "DHH"
+        Customer.user = "Rafael"
+
+    Before:
+
+        Account.user  # => "Rafael"
+
+    After:
+
+        Account.user  # => "DHH"
+
+    *Shinichi Maeshima*
+
 *   Since weeks are no longer converted to days, add `:weeks` to the list of
     parts that `ActiveSupport::TimeWithZone` will recognize as possibly being
     of variable duration to take account of DST transitions.
diff --git a/activesupport/lib/active_support/core_ext/module/attribute_accessors_per_thread.rb b/activesupport/lib/active_support/core_ext/module/attribute_accessors_per_thread.rb
index 1ebae1f4f3568a00bc98ba11b2d64c79043443a5..a40f0df9e250fb7b77c8dbb23a1c3aea7a4a528a 100644
--- a/activesupport/lib/active_support/core_ext/module/attribute_accessors_per_thread.rb
+++ b/activesupport/lib/active_support/core_ext/module/attribute_accessors_per_thread.rb
@@ -41,14 +41,14 @@ def thread_mattr_reader(*syms)
       raise NameError.new("invalid attribute name: #{sym}") unless /^[_A-Za-z]\w*$/.match?(sym)
       class_eval(<<-EOS, __FILE__, __LINE__ + 1)
         def self.#{sym}
-          Thread.current[:"attr_#{name}_#{sym}"]
+          Thread.current["attr_"+ name + "_#{sym}"]
         end
       EOS
 
       unless options[:instance_reader] == false || options[:instance_accessor] == false
         class_eval(<<-EOS, __FILE__, __LINE__ + 1)
           def #{sym}
-            Thread.current[:"attr_#{name}_#{sym}"]
+            Thread.current["attr_"+ self.class.name + "_#{sym}"]
           end
         EOS
       end
@@ -80,14 +80,14 @@ def thread_mattr_writer(*syms)
       raise NameError.new("invalid attribute name: #{sym}") unless /^[_A-Za-z]\w*$/.match?(sym)
       class_eval(<<-EOS, __FILE__, __LINE__ + 1)
         def self.#{sym}=(obj)
-          Thread.current[:"attr_#{name}_#{sym}"] = obj
+          Thread.current["attr_"+ name + "_#{sym}"] = obj
         end
       EOS
 
       unless options[:instance_writer] == false || options[:instance_accessor] == false
         class_eval(<<-EOS, __FILE__, __LINE__ + 1)
           def #{sym}=(obj)
-            Thread.current[:"attr_#{name}_#{sym}"] = obj
+            Thread.current["attr_"+ self.class.name + "_#{sym}"] = obj
           end
         EOS
       end
diff --git a/activesupport/test/core_ext/module/attribute_accessor_per_thread_test.rb b/activesupport/test/core_ext/module/attribute_accessor_per_thread_test.rb
index cda5a344422ba52ade4e5b720ec74a536062021e..b816fa50e35676a8825d89bd78c680398d5e488e 100644
--- a/activesupport/test/core_ext/module/attribute_accessor_per_thread_test.rb
+++ b/activesupport/test/core_ext/module/attribute_accessor_per_thread_test.rb
@@ -8,6 +8,12 @@ def setup
       thread_mattr_accessor :bar,  instance_writer: false
       thread_mattr_reader   :shaq, instance_reader: false
       thread_mattr_accessor :camp, instance_accessor: false
+
+      def self.name; "MyClass" end
+    end
+
+    @subclass = Class.new(@class) do
+      def self.name; "SubMyClass" end
     end
 
     @object = @class.new
@@ -112,4 +118,14 @@ def test_should_return_same_value_by_class_or_instance_accessor
 
     assert_equal @class.foo, @object.foo
   end
+
+  def test_should_not_affect_superclass_if_subclass_set_value
+    @class.foo = "super"
+    assert_equal @class.foo, "super"
+    assert_nil @subclass.foo
+
+    @subclass.foo = "sub"
+    assert_equal @class.foo, "super"
+    assert_equal @subclass.foo, "sub"
+  end
 end