Seal Island is an island in Investigator Strait off the south coast of Yorke Peninsula in the Australian state of South Australia, about 7 kilometers south-southwest of Stenhouse Bay. It has been a member of the Althorpe Islands Conservation Park since 1972. Seal Island originated between 7500 and 8900 years ago as sea levels increased at the beginning of the Holocene, separating Yorke Peninsula from Kangaroo Island. A survey conducted in 1996 revealed the presence of ten plant species. Latest surveys discovered that Australian sea lions utilize the island as a haul-out spot and possibly as a rookery, and the Crested tern has been observed on the island.