Inner Sister Island, part of the Sister Islands Conservation Area, is a granite and dolerite island, with an area of 748 hectares (2.89 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, pied oystercatcher and sooty oystercatcher.

Resident reptiles are the three-lined skink, White's skink, white-lipped snake and tiger snake.

Apart from the sheep, mammals present on the island include the eastern barred bandicoot, Tasmanian pademelon and the introduced European hare.