Terastia subjectalis is a moth of the Crambidae family. It occurs across the Indian and south Pacific Oceans, including Fiji, Hawaii, Réunion, Okinawa, Samoa, the Society Islands, Sri Lanka and Australia (the Northern Territory, Queensland and Western Australia). Adults have wavy wings, with a pattern of light and dark brown on each forewing. The hindwings are pale brown with a broad darker margin. The larvae feed on Erythrina species, including Erythrina monosperma and Erythrina indica. The larvae feed in the pods but also bore in the shoots.