Ennomos erosaria, the September thorn, is a moth of the family Geometridae. The species can be found in the Palearctic ecozone in West Europe and from central Scandinavia its range extends to the Northern Mediterranean and East to the Caucasus and Russia.

The wingspan is 30–35 mm. The length of the forewings is 17–21 mm. The ground colour is straw yellow to ochre-yellow. The colour is darker on hindwing termen and the hindwing also has a faint discal spot .The fasciae (bands) are brown.The inward arc under projection on distal forewing is prominent.

Similar species: Other Ennomos species.

The moths fly in one generation from July to October. They are attracted to light.

The caterpillar, which resembles a small twig, feeds on oak, birch, and lime.