diff --git a/ext/psych/lib/psych/visitors/yaml_tree.rb b/ext/psych/lib/psych/visitors/yaml_tree.rb
index 4abdda31d6d2f8..318f5f892b28b7 100644
--- a/ext/psych/lib/psych/visitors/yaml_tree.rb
+++ b/ext/psych/lib/psych/visitors/yaml_tree.rb
@@ -267,7 +267,7 @@ def visit_String o
           tag   = 'tag:yaml.org,2002:str'
           plain = false
           quote = false
-        elsif o == 'y' || o == 'n'
+        elsif o == 'y' || o == 'Y' || o == 'n' || o == 'N'
           style = Nodes::Scalar::DOUBLE_QUOTED
         elsif @line_width && o.length > @line_width
           style = Nodes::Scalar::FOLDED
diff --git a/test/psych/test_string.rb b/test/psych/test_string.rb
index 0dc34b3083d641..c996a8fc2dc197 100644
--- a/test/psych/test_string.rb
+++ b/test/psych/test_string.rb
@@ -24,10 +24,19 @@ def initialize
     # "ambiguity" in the emitted document
     def test_y_is_quoted
       assert_match(/"y"/, Psych.dump("y"))
+      assert_match(/"Y"/, Psych.dump("Y"))
     end
 
     def test_n_is_quoted
       assert_match(/"n"/, Psych.dump("n"))
+      assert_match(/"N"/, Psych.dump("N"))
+    end
+
+    def test_all_yaml_1_1_booleans_are_quoted
+      yaml_1_1_booleans = %w[y Y yes Yes YES n N no No NO true True TRUE false False FALSE on On ON off Off OFF] # from https://yaml.org/type/bool.html
+      yaml_1_1_booleans.each do |boolean|
+        assert_match(/"#{boolean}"|'#{boolean}'/, Psych.dump(boolean))
+      end
     end
 
     def test_string_with_newline