Triangular Butterflyfish

Chaetodon triangulum has a flat triangular shaped body, emphasised when the fins are fully spread, and an elongated snout. Its body is predominantly whitish in colour with many broad, vertical grey chevron-shaped bands along the sides, Its mouth is orange, and there is a brownish orange vertical band through the eye, which grows brighter orange closer to the top of the head widening as it extends rearward on the base of the dorsal fin in adults. The caudal peduncle is black, as is the caudal fin but its margins are yellow. The dorsal fin has 11–12 spines and 23–26 soft rays while the anal fin contains 3 spines and 20–21 soft rays. This species attains a maximum total length of 16 centimetres (6.3 in).

Chaetodon triangulum has a wide distribution in the Indian Ocean from the coast of tropical East Africa east to the Andaman Sea and along the Indian Ocean coastlines of Sumatra and Java. It also appears at the Seribu Islands and has been reported near Bali.