diff --git a/stdlib/psych/0/core_ext.rbs b/stdlib/psych/0/core_ext.rbs
index a7f9e72852..0aa68795de 100644
--- a/stdlib/psych/0/core_ext.rbs
+++ b/stdlib/psych/0/core_ext.rbs
@@ -1,5 +1,7 @@
 %a{annotate:rdoc:skip}
 class Object
+  def self.yaml_tag: (String url) -> String
+
   # <!--
   #   rdoc-file=ext/psych/lib/psych/core_ext.rb
   #   - to_yaml(options = {})
diff --git a/test/stdlib/psych_test.rb b/test/stdlib/psych_test.rb
index 4af1c990e7..98d7435d1b 100644
--- a/test/stdlib/psych_test.rb
+++ b/test/stdlib/psych_test.rb
@@ -129,3 +129,20 @@ def test_to_yaml
     )
   end
 end
+
+class PsychObjectSingletonTest < Test::Unit::TestCase
+  include TestHelper
+
+  library "psych"
+  testing "singleton(::Object)"
+
+  class Tagged
+  end
+
+  def test_yaml_tag
+    assert_send_type(
+      "(::String) -> ::String",
+      Tagged, :yaml_tag, "!rbs-test/tagged"
+    )
+  end
+end