diff --git a/lib/ruby_lsp/server.rb b/lib/ruby_lsp/server.rb
index 170c755da..6a1956d2c 100644
--- a/lib/ruby_lsp/server.rb
+++ b/lib/ruby_lsp/server.rb
@@ -120,6 +120,16 @@ def process_message(message)
         @global_state.synchronize { @cancelled_requests << message[:params][:id] }
       when nil
         process_response(message) if message[:result]
+      else
+        id = message[:id]
+
+        if id
+          send_message(Error.new(
+            id: id,
+            code: Constant::ErrorCodes::METHOD_NOT_FOUND,
+            message: "Method not found: #{message[:method]}",
+          ))
+        end
       end
     rescue DelegateRequestError
       send_message(Error.new(id: message[:id], code: DelegateRequestError::CODE, message: "DELEGATE_REQUEST"))
diff --git a/test/server_test.rb b/test/server_test.rb
index f38c20fba..12b6a0e5a 100644
--- a/test/server_test.rb
+++ b/test/server_test.rb
@@ -1726,6 +1726,19 @@ def test_launch_bundle_compose_forwards_argv_to_launcher
     ARGV.replace(original_argv)
   end
 
+  def test_unrecognized_request_returns_method_not_found
+    non_existent_method = "textDocument/nonExistentMethod"
+    @server.process_message({
+      id: 1,
+      method: non_existent_method,
+      params: {},
+    })
+
+    error = find_message(RubyLsp::Error)
+    assert_equal(Constant::ErrorCodes::METHOD_NOT_FOUND, error.code)
+    assert_equal("Method not found: #{non_existent_method}", error.message)
+  end
+
   private
 
   def run_initialize_server_with_setup_error(error)