Theila siennata is a moth in the Crambidae family. It was described by Warren in 1896. It is found in Australia, where it has been recorded from Queensland. The species has been suggested as a biological control agent in the United States. The forewings are white with brown markings and orange margins. The hindwings are white with a broad orange margin, with some black dots and white patches. The larvae feed on the leaves of Hydrilla species.