Swainson Island is an unpopulated island with an adjacent islet, located close to the south-western coast of Tasmania, Australia.

Situated near where the mouth of Port Davey meets the Southern Ocean, the 4.14-hectare (10.2-acre) island is part of the Swainson Islands Group, and comprises part of the Southwest National Park and the Tasmanian Wilderness World Heritage Site.

[1][2] The island is part of the Port Davey Islands Important Bird Area, so identified by BirdLife International because of its importance for breeding seabirds.

[3] Recorded breeding seabird and wader species are the little penguin (100 pairs), short-tailed shearwater (38,000 pairs), fairy prion (3000 pairs), Pacific gull and sooty oystercatcher.

The Tasmanian tree skink is present.