diff --git a/lib/steep/source.rb b/lib/steep/source.rb
index a77a1da45..64a2e542a 100644
--- a/lib/steep/source.rb
+++ b/lib/steep/source.rb
@@ -459,12 +459,14 @@ def self.insert_type_node(node, comments)
           case node.type
           when :lvasgn, :ivasgn, :gvasgn, :cvasgn, :casgn
             # Skip
+          when :return, :break, :next
+            # Skip
+          when :def, :defs
+            # Skip
           when :masgn
             lhs, rhs = node.children
             node = node.updated(nil, [lhs, insert_type_node(rhs, comments)])
             return adjust_location(node)
-          when :return, :break, :next
-            # Skip
           when :begin
             location = node.loc #: Parser::Source::Map & Parser::AST::_Collection
             if location.begin
@@ -549,6 +551,21 @@ def self.insert_type_node(node, comments)
             ]
           )
         )
+      when :def
+        name, args, body = node.children
+        assertion_location = args&.location&.expression || (_ = node.location).name
+        no_assertion_comments = comments.except(assertion_location.last_line)
+        args = insert_type_node(args, no_assertion_comments)
+        body = insert_type_node(body, comments) if body
+        return adjust_location(node.updated(nil, [name, args, body]))
+      when :defs
+        object, name, args, body = node.children
+        assertion_location = args&.location&.expression || (_ = node.location).name
+        no_assertion_comments = comments.except(assertion_location.last_line)
+        object = insert_type_node(object, no_assertion_comments)
+        args = insert_type_node(args, no_assertion_comments)
+        body = insert_type_node(body, comments) if body
+        return adjust_location(node.updated(nil, [object, name, args, body]))
       else
         adjust_location(
           map_child_node(node, nil) {|child| insert_type_node(child, comments) }
diff --git a/sig/steep/source.rbs b/sig/steep/source.rbs
index 6c1ce0927..15f92be6e 100644
--- a/sig/steep/source.rbs
+++ b/sig/steep/source.rbs
@@ -94,6 +94,8 @@ module Steep
 
     def self.adjust_location: (Parser::AST::Node) -> Parser::AST::Node
 
+    # Returns an `:assertion` node with `TypeAssertion`
+    # 
     def self.assertion_node: (Parser::AST::Node, AST::Node::TypeAssertion) -> Parser::AST::Node
 
     def self.type_application_node: (Parser::AST::Node, AST::Node::TypeApplication) -> Parser::AST::Node
diff --git a/test/source_test.rb b/test/source_test.rb
index 6c0d782f6..6ad5dd2fc 100644
--- a/test/source_test.rb
+++ b/test/source_test.rb
@@ -712,6 +712,73 @@ def test_assertion__no_skip
     end
   end
 
+  def test_assertion__def
+    with_factory do |factory|
+      source = Steep::Source.parse(<<~RUBY, path: Pathname("foo.rb"), factory: factory)
+        def foo(x) #: String
+        end
+
+        def bar #: void
+        end
+
+        def baz(x) = 123 #: Numeric
+      RUBY
+
+      source.node.children[0].tap do |node|
+        assert_equal :def, node.type
+        assert_equal :args, dig(node, 1).type
+        assert_equal :arg, dig(node, 1, 0).type
+        assert_nil dig(node, 2)
+      end
+      source.node.children[1].tap do |node|
+        assert_equal :def, node.type
+        assert_equal :args, dig(node, 1).type
+        assert_nil dig(node, 2)
+      end
+      source.node.children[2].tap do |node|
+        assert_equal :def, node.type
+        assert_equal :args, dig(node, 1).type
+        assert_equal :arg, dig(node, 1, 0).type
+        assert_equal :assertion, dig(node, 2).type
+      end
+    end
+  end
+
+  def test_assertion__defs
+    with_factory do |factory|
+      source = Steep::Source.parse(<<~RUBY, path: Pathname("foo.rb"), factory: factory)
+        def self.foo(x) #: String
+        end
+
+        def self.bar #: void
+        end
+
+        def self.baz(x) = 123 #: Numeric
+      RUBY
+
+      source.node.children[0].tap do |node|
+        assert_equal :defs, node.type
+        assert_equal :self, dig(node, 0).type
+        assert_equal :args, dig(node, 2).type
+        assert_equal :arg, dig(node, 2, 0).type
+        assert_nil dig(node, 3)
+      end
+      source.node.children[1].tap do |node|
+        assert_equal :defs, node.type
+        assert_equal :self, dig(node, 0).type
+        assert_equal :args, dig(node, 2).type
+        assert_nil dig(node, 3)
+      end
+      source.node.children[2].tap do |node|
+        assert_equal :defs, node.type
+        assert_equal :self, dig(node, 0).type
+        assert_equal :args, dig(node, 2).type
+        assert_equal :arg, dig(node, 2, 0).type
+        assert_equal :assertion, dig(node, 3).type
+      end
+    end
+  end
+
   def test_tapp_send
     with_factory({ Pathname("foo.rbs") => <<-RBS }) do |factory|
 class Tapp
diff --git a/test/type_check_test.rb b/test/type_check_test.rb
index 68783f0ad..99310de85 100644
--- a/test/type_check_test.rb
+++ b/test/type_check_test.rb
@@ -2077,4 +2077,30 @@ def f(&block)
       YAML
     )
   end
+
+  def test_args_annotation
+    run_type_check_test(
+      signatures: {
+        "a.rbs" => <<~RBS
+          class Hello
+            def foo: (String) -> Integer
+          end
+        RBS
+      },
+      code: {
+        "a.rb" => <<~RUBY
+          class Hello
+            def foo(x) #: Integer
+              3
+            end
+          end
+        RUBY
+      },
+      expectations: <<~YAML
+        ---
+        - file: a.rb
+          diagnostics: []
+      YAML
+    )
+  end
 end