Outer Sister Island, part of the Sister Islands Conservation Area, is a granite and dolerite island, with an area of 545 hectares (2.10 sq mi), located in Bass Strait, Tasmania, Australia.

The island is grazed by sheep and annual muttonbirding takes place.

[1] Seabirds and waders recorded as breeding on the island include little penguin, short-tailed shearwater, silver gull, Pacific gull and sooty oystercatcher.

Resident reptiles are the metallic skink, White's skink, white-lipped snake, lowland copperhead and tiger snake.

Apart from the sheep, mammals present on the island include the Tasmanian pademelon and the introduced house mouse, as well as feral cats.