Thistle Island is in the Spencer Gulf, South Australia, some 200 km west of Adelaide, and northwest of the Gambier Islands. The town of Port Lincoln lies to the northwest of the island. Between them, the Gambier Islands and Thistle form a chain across the mouth of the gulf between the southern tips of the Yorke and Eyre Peninsulas. As of 2014 the Royal Australian Navy operated an acoustic range in the waters west of Thistle Island, with a small control facility being located on the island itself.
Coordinates: 34°59′56″S 136°08′30″E / 34.998824°S 136.141641°E