The functional composition of reef fish assemblages is highly conserved across large biogeographic areas, but it is unknown whether assembly rules hold at biogeographical and environmental extremes for coral reefs. This study examined the functional composition of butterflyfishes in the Persian Gulf, Musandam Peninsula, and Gulf of Oman. Only five species of butterflyfishes were recorded during this study, and mostly just in the Gulf of Oman. Unlike most locations in the Indo–Pacific where butterflyfish assemblages are dominated by obligate corallivores, the only obligate corallivore recorded, Chaetodon melapterus, was rare or absent at all locations. The most common and widespread species was Chaetodon nigropunctatus, which is shown to be a facultative corallivore. The diversity of butterflyfishes in the Persian Gulf is likely to have been constrained by its’ biogeographical history and isolation, but functional composition appears to be further affected by limited abundance of prey corals and harsh environmental conditions.