The purple thorn (Selenia tetralunaria) is a moth of the family Geometridae.

The margins of the wings are angular, giving the species a distinctive shape.

Two broods are produced each year and they differ considerably in size and colour.

Adults of the spring brood, which fly in April and May, have a wingspan of 46–52 mm.

Moths of the summer brood, flying in July and August,[1] are smaller (wingspan 44–46 mm) and darker.