The purple thorn (Selenia tetralunaria) is a moth of the family Geometridae. It is a species of northern and central Europe. It has a scattered distribution in Britain but is absent from Ireland.
This species is dark purplish-brown, paler towards the termen, with a dark spot at the apex of the forewing. Both wings are marked with a comma-shaped white mark. 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, are smaller (wingspan 44–46 mm) and darker. The species flies at night and is attracted to light.