Chaetodon triangulum has a flat triangular shaped body, emphasised when the fins are fully spread, and an elongated snout.

This is a territorial species which lives in pairs, although solitary juveniles hide among coral branches.

[2] Triangle butterflyfish are obligate corallivores that feed primarily on Acropora corals.

They defend patches of these corals and actively chase other butterflyfishes from these territories and the food supply it contains.

[1] Chaetodon triangulum was first formally described in 1831 by the French anatomist Georges Cuvier (1769-1832) with the type locality given as Jakarta.