diff --git a/src/Typeclass/Monoid.php b/src/Typeclass/Monoid.php
new file mode 100644
index 0000000..20bb7f1
--- /dev/null
+++ b/src/Typeclass/Monoid.php
@@ -0,0 +1,20 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Marcosh\LamPHPda\Typeclass;
+
+/**
+ * @psalm-immutable
+ *
+ * @template A
+ */
+interface Monoid extends Semigroup
+{
+    /**
+     * @psalm-pure
+     *
+     * @return Monoid<A>
+     */
+    public static function mempty(): Monoid;
+}