diff --git a/CHANGES.md b/CHANGES.md
index 0895827c0..8b94c614e 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -2,6 +2,8 @@
 
 ### Unreleased
 
+* Fix `JSON.generate` `strict: true` mode to also restrict hash keys.
+
 ### 2025-07-28 (2.13.2)
 
 * Improve duplicate key warning and errors to include the key name and point to the right caller.
diff --git a/ext/json/ext/generator/generator.c b/ext/json/ext/generator/generator.c
index 9c6ed9304..701bb41b8 100644
--- a/ext/json/ext/generator/generator.c
+++ b/ext/json/ext/generator/generator.c
@@ -1041,6 +1041,9 @@ json_object_i(VALUE key, VALUE val, VALUE _arg)
             key_to_s = rb_sym2str(key);
             break;
         default:
+            if (data->state->strict) {
+                raise_generator_error(key, "%"PRIsVALUE" not allowed in JSON", rb_funcall(key, i_to_s, 0));
+            }
             key_to_s = rb_convert_type(key, T_STRING, "String", "to_s");
             break;
     }
diff --git a/java/src/json/ext/Generator.java b/java/src/json/ext/Generator.java
index 852509205..19b615883 100644
--- a/java/src/json/ext/Generator.java
+++ b/java/src/json/ext/Generator.java
@@ -544,6 +544,9 @@ private static void processEntry(ThreadContext context, Session session, OutputS
             } else if (keyClass == runtime.getSymbol()) {
                 keyStr = ((RubySymbol) key).id2name(context);
             } else {
+                if (session.getState(context).strict()) {
+                    throw Utils.buildGeneratorError(context, key, key + " not allowed in JSON").toThrowable();
+                }
                 keyStr = TypeConverter.convertToType(key, runtime.getString(), "to_s");
             }
 
diff --git a/lib/json/truffle_ruby/generator.rb b/lib/json/truffle_ruby/generator.rb
index aba20068a..937cad68b 100644
--- a/lib/json/truffle_ruby/generator.rb
+++ b/lib/json/truffle_ruby/generator.rb
@@ -476,6 +476,10 @@ def json_transform(state)
               result << delim unless first
               result << state.indent * depth if indent
 
+              if state.strict? && !(Symbol === key || String === key)
+                raise GeneratorError.new("#{key.class} not allowed in JSON", value)
+              end
+
               key_str = key.to_s
               if key_str.is_a?(String)
                 key_json = key_str.to_json(state)
diff --git a/test/json/json_generator_test.rb b/test/json/json_generator_test.rb
index 914b3f4ed..963350ea4 100755
--- a/test/json/json_generator_test.rb
+++ b/test/json/json_generator_test.rb
@@ -404,6 +404,18 @@ def test_json_generate_unsupported_types
     assert_raise JSON::GeneratorError do
       generate(Object.new, strict: true)
     end
+
+    assert_raise JSON::GeneratorError do
+      generate([Object.new], strict: true)
+    end
+
+    assert_raise JSON::GeneratorError do
+      generate({ "key" => Object.new }, strict: true)
+    end
+
+    assert_raise JSON::GeneratorError do
+      generate({ Object.new => "value" }, strict: true)
+    end
   end
 
   def test_nesting