The fauna and population densities of butterflyfish (Chaetodontidae: Perciformes) were studied at the Cobourg Marine Park (CMP), Cobourg Peninsula in the Arafura Sea, the Northern Territory, Australia. A total of five genera and 17 species of butterflyfish were recorded within the CMP, including nine new records for the Northern Territory. Goldstripe butterflyfish Chaetodon aureofasciatus Macleay, the most common and abundant species represented 64.3% of all 883 butterflyfish recorded followed by the Philippine butterflyfish Chaetodon adiergastos Seale (14.9%), margined butterflyfish Chelmon marginalis Richardson (10.8%), triangle butterflyfish Chaetodon baronessa Cuvier (5.1%) and chevroned butterflyfish C. trifascialis Quoy and Gaimard (1.2%). The CMP contains the richest chaetodontid fauna among all sites surveyed so far in the Northern Territory. Population density of the endemic Australian species Chelmon marginalis in the CMP is the highest across its range within studied locations. Abundances of Chaetodon adiergastos at Cobourg are similar to areas in the Philippines and Malaysia where population density of this species is the highest in the Indo-West Pacific region.