Lythria purpuraria, the purple-barred yellow, is a species of moth of the family Geometridae.

It is found from western Europe to Siberia, Russia, Ukraine, Turkmenistan and Kazakhstan.

L. purpuraria generally have two red-purple transverse lines in their green-yellow forewings.

[1] L. purpuraria and the other four species within the Lythria family have a very difficult genitalia structure, which is complicated to analyze and study.

The larvae feed on prostrate knotweed, also known as Polygonum aviculare.