It is the second largest island in South Australia and about 13 km (8.1 mi) long.

[1] It was one of the first parts of South Australia to be discovered and named by Europeans, along with St Francis Island, mapped by François Thijssen in 't Gulden Zeepaert in 1627.

The historic St Peter Island Whaling Sites are listed on the South Australian Heritage Register as a designated place of archaeological significance.

[4][5][6] The island is part of the Nuyts Archipelago Important Bird Area (IBA), so identified by BirdLife International because it supports over 1% of the world populations of short-tailed shearwaters, white-faced storm-petrels and pied oystercatchers.

[7] The island lies across the Yatala Channel from the separate Tourville and Murat Bays Important Bird Area, just west of Ceduna on the mainland.