diff --git a/ChangeLog b/ChangeLog
index a44ab6c6e4..28621c3d18 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Thu Dec 31 14:36:45 2015  Marc-Andre Lafortune  <ruby-core@marc-andre.ca>
+
+	* lib/ostruct.rb: Fix new_ostruct_member to correctly avoid
+	  redefinition [#11901]
+
 Thu Dec 31 02:45:12 2015  NARUSE, Yui  <naruse@ruby-lang.org>
 
 	* test/ruby/test_module.rb (test_classpath): r53376 may change
diff --git a/lib/ostruct.rb b/lib/ostruct.rb
index e7b8ed4609..e5b69608ad 100644
--- a/lib/ostruct.rb
+++ b/lib/ostruct.rb
@@ -168,7 +168,7 @@ class OpenStruct
   #
   def new_ostruct_member(name)
     name = name.to_sym
-    unless respond_to?(name)
+    unless singleton_class.method_defined?(name)
       define_singleton_method(name) { @table[name] }
       define_singleton_method("#{name}=") { |x| modifiable[name] = x }
     end
diff --git a/test/ostruct/test_ostruct.rb b/test/ostruct/test_ostruct.rb
index 8b0424d2d8..02c4c74152 100644
--- a/test/ostruct/test_ostruct.rb
+++ b/test/ostruct/test_ostruct.rb
@@ -164,4 +164,21 @@ class TC_OpenStruct < Test::Unit::TestCase
     e = assert_raise(ArgumentError) { os.send :foo=, true, true }
     assert_match(/#{__callee__}/, e.backtrace[0])
   end
+
+  def test_accessor_defines_method
+    os = OpenStruct.new(foo: 42)
+    assert os.respond_to? :foo
+    assert_equal([], os.singleton_methods)
+    assert_equal(42, os.foo)
+    assert_equal([:foo, :foo=], os.singleton_methods)
+  end
+
+  def test_does_not_redefine
+    os = OpenStruct.new(foo: 42)
+    def os.foo
+      43
+    end
+    os.foo = 44
+    assert_equal(43, os.foo)
+  end
 end