diff --git a/ext/rbs_extension/constants.c b/ext/rbs_extension/constants.c
index 57ec73ee3..0e21a1860 100644
--- a/ext/rbs_extension/constants.c
+++ b/ext/rbs_extension/constants.c
@@ -61,6 +61,7 @@ VALUE RBS_Types_ClassInstance;
 VALUE RBS_Types_ClassSingleton;
 VALUE RBS_Types_Function_Param;
 VALUE RBS_Types_Function;
+VALUE RBS_Types_UntypedFunction;
 VALUE RBS_Types_Interface;
 VALUE RBS_Types_Intersection;
 VALUE RBS_Types_Literal;
@@ -138,6 +139,7 @@ void rbs__init_constants(void) {
   IMPORT_CONSTANT(RBS_Types_ClassInstance, RBS_Types, "ClassInstance");
   IMPORT_CONSTANT(RBS_Types_ClassSingleton, RBS_Types, "ClassSingleton");
   IMPORT_CONSTANT(RBS_Types_Function, RBS_Types, "Function");
+  IMPORT_CONSTANT(RBS_Types_UntypedFunction, RBS_Types, "UntypedFunction");
   IMPORT_CONSTANT(RBS_Types_Function_Param, RBS_Types_Function, "Param");
   IMPORT_CONSTANT(RBS_Types_Interface, RBS_Types, "Interface");
   IMPORT_CONSTANT(RBS_Types_Intersection, RBS_Types, "Intersection");
diff --git a/ext/rbs_extension/constants.h b/ext/rbs_extension/constants.h
index 978bc32d0..2d1ca63bf 100644
--- a/ext/rbs_extension/constants.h
+++ b/ext/rbs_extension/constants.h
@@ -64,6 +64,7 @@ extern VALUE RBS_Types_ClassInstance;
 extern VALUE RBS_Types_ClassSingleton;
 extern VALUE RBS_Types_Function_Param;
 extern VALUE RBS_Types_Function;
+extern VALUE RBS_Types_UntypedFunction;
 extern VALUE RBS_Types_Interface;
 extern VALUE RBS_Types_Intersection;
 extern VALUE RBS_Types_Literal;
diff --git a/ext/rbs_extension/parser.c b/ext/rbs_extension/parser.c
index f040a1e86..04fc79a80 100644
--- a/ext/rbs_extension/parser.c
+++ b/ext/rbs_extension/parser.c
@@ -52,6 +52,10 @@ typedef struct {
   VALUE rest_keywords;
 } method_params;
 
+static bool rbs_is_untyped_params(method_params *params) {
+  return NIL_P(params->required_positionals);
+}
+
 // /**
 //  * Returns RBS::Location object of `current_token` of a parser state.
 //  *
@@ -362,6 +366,7 @@ static bool is_keyword(parserstate *state) {
 
 /*
   params ::= {} `)`
+           | {} `?` `)`               -- Untyped function params (assign params.required = nil)
            | <required_params> `)`
            | <required_params> `,` `)`
 
@@ -389,6 +394,11 @@ static bool is_keyword(parserstate *state) {
              | {} `**` <function_param>
 */
 static void parse_params(parserstate *state, method_params *params) {
+  if (state->next_token.type == pQUESTION && state->next_token2.type == pRPAREN) {
+    params->required_positionals = Qnil;
+    parser_advance(state);
+    return;
+  }
   if (state->next_token.type == pRPAREN) {
     return;
   }
@@ -613,6 +623,13 @@ static void parse_function(parserstate *state, VALUE *function, VALUE *block, VA
     parser_advance_assert(state, pRPAREN);
   }
 
+  // Untyped method parameter means it cannot have block
+  if (rbs_is_untyped_params(&params)) {
+    if (state->next_token.type != pARROW) {
+      raise_syntax_error(state, state->next_token2, "A method type with untyped method parameter cannot have block");
+    }
+  }
+
   // Passing NULL to function_self_type means the function itself doesn't accept self type binding. (== method type)
   if (function_self_type) {
     *function_self_type = parse_self_type_binding(state);
@@ -641,8 +658,11 @@ static void parse_function(parserstate *state, VALUE *function, VALUE *block, VA
     parser_advance_assert(state, pARROW);
     VALUE block_return_type = parse_optional(state);
 
-    *block = rbs_block(
-      rbs_function(
+    VALUE block_function = Qnil;
+    if (rbs_is_untyped_params(&block_params)) {
+      block_function = rbs_untyped_function(block_return_type);
+    } else {
+      block_function = rbs_function(
         block_params.required_positionals,
         block_params.optional_positionals,
         block_params.rest_positionals,
@@ -651,10 +671,10 @@ static void parse_function(parserstate *state, VALUE *function, VALUE *block, VA
         block_params.optional_keywords,
         block_params.rest_keywords,
         block_return_type
-      ),
-      required,
-      block_self_type
-    );
+      );
+    }
+
+    *block = rbs_block(block_function, required, block_self_type);
 
     parser_advance_assert(state, pRBRACE);
   }
@@ -662,16 +682,20 @@ static void parse_function(parserstate *state, VALUE *function, VALUE *block, VA
   parser_advance_assert(state, pARROW);
   VALUE type = parse_optional(state);
 
-  *function = rbs_function(
-    params.required_positionals,
-    params.optional_positionals,
-    params.rest_positionals,
-    params.trailing_positionals,
-    params.required_keywords,
-    params.optional_keywords,
-    params.rest_keywords,
-    type
-  );
+  if (rbs_is_untyped_params(&params)) {
+    *function = rbs_untyped_function(type);
+  } else {
+    *function = rbs_function(
+      params.required_positionals,
+      params.optional_positionals,
+      params.rest_positionals,
+      params.trailing_positionals,
+      params.required_keywords,
+      params.optional_keywords,
+      params.rest_keywords,
+      type
+    );
+  }
 }
 
 /*
diff --git a/ext/rbs_extension/ruby_objs.c b/ext/rbs_extension/ruby_objs.c
index 2a92d1c56..0e9760ebb 100644
--- a/ext/rbs_extension/ruby_objs.c
+++ b/ext/rbs_extension/ruby_objs.c
@@ -170,6 +170,17 @@ VALUE rbs_function_param(VALUE type, VALUE name, VALUE location) {
   );
 }
 
+VALUE rbs_untyped_function(VALUE return_type) {
+  VALUE args = rb_hash_new();
+  rb_hash_aset(args, ID2SYM(rb_intern("return_type")), return_type);
+
+  return CLASS_NEW_INSTANCE(
+    RBS_Types_UntypedFunction,
+    1,
+    &args
+  );
+}
+
 VALUE rbs_function(
   VALUE required_positional_params,
   VALUE optional_positional_params,
diff --git a/ext/rbs_extension/ruby_objs.h b/ext/rbs_extension/ruby_objs.h
index d9ee32262..bf5ea0763 100644
--- a/ext/rbs_extension/ruby_objs.h
+++ b/ext/rbs_extension/ruby_objs.h
@@ -30,6 +30,7 @@ VALUE rbs_class_instance(VALUE typename, VALUE type_args, VALUE location);
 VALUE rbs_class_singleton(VALUE typename, VALUE location);
 VALUE rbs_function_param(VALUE type, VALUE name, VALUE location);
 VALUE rbs_function(VALUE required_positional_params, VALUE optional_positional_params, VALUE rest_positional_params, VALUE trailing_positional_params, VALUE required_keywords, VALUE optional_keywords, VALUE rest_keywords, VALUE return_type);
+VALUE rbs_untyped_function(VALUE return_type);
 VALUE rbs_interface(VALUE typename, VALUE type_args, VALUE location);
 VALUE rbs_intersection(VALUE types, VALUE location);
 VALUE rbs_literal(VALUE literal, VALUE location);
diff --git a/test/rbs/parser_test.rb b/test/rbs/parser_test.rb
index ecd5b6c01..a29d992ac 100644
--- a/test/rbs/parser_test.rb
+++ b/test/rbs/parser_test.rb
@@ -751,4 +751,20 @@ def test_parse_require_eof
       RBS::Parser.parse_method_type("() -> void () -> void", range: 0..., require_eof: true)
     end
   end
+
+  def test_proc__untyped_function
+    RBS::Parser.parse_type("^(?) -> Integer").tap do |type|
+      assert_instance_of RBS::Types::UntypedFunction, type.type
+    end
+
+    RBS::Parser.parse_type("^() { (?) -> String } -> Integer").tap do |type|
+      assert_instance_of RBS::Types::UntypedFunction, type.block.type
+    end
+  end
+
+  def test_proc__untyped_function_parse_error
+    assert_raises(RBS::ParsingError) do
+      RBS::Parser.parse_type("^(?) { (?) -> void } -> Integer")
+    end
+  end
 end