Squaxin Island is in the extreme southwestern part of Puget Sound in Mason County, Washington, United States. The island is an Indian reservation of the Native American Squaxin Island Tribe. A Washington state park on the island, by the same name, was closed and the land was returned to the Squaxin Tribe. The island's land area is 5.739 km² (2.216 sq mi). There was no resident population as of the 2000 census.

Squaxin Island is separated from Harstine Island, to the east, by Peale Passage.

The island's name comes from the Lushootseed place–name sqʷax̌səd.^[1]