Ennomos erosaria, the September thorn, is a moth of the family Geometridae.

The species can be found in the Palearctic realm in western Europe and from central Scandinavia.

Its range extends to the northern Mediterranean and east to the Caucasus and Russia.

The inward arc under projection on distal forewing is prominent.The name typical form is yellowish.

The larva which is rather elongate, with strong knob-shaped protuberances, altogether bearing a remarkable likeness to an oak twig is brown, now with a tinge of purple, now of greenish, the ventral surface paler greenish brown.It feeds on oak, birch, and lime.