Big Black Reef is a small, flat dolerite island, with an area of 0.54 ha, in south-eastern Australia. It is part of Tasmania’s Long Island Group, lying in eastern Bass Strait west of Cape Barren Island in the Furneaux Group. Together with nearby Boxen Island it is classified by BirdLife International as an Important Bird Area because it has been recorded as supporting 288 breeding pairs of black-faced cormorants.