Anderson Island is the southernmost island in Puget Sound and a census-designated place of Pierce County, Washington, United States. It is accessible by boat or a 20-minute ferry ride from Steilacoom. Anderson Island lies just south of McNeil Island. To the northwest Key Peninsula lies across Drayton Passage. The south basin of Puget Sound separates the island from the mainland to the southeast, while to the southwest the Nisqually Reach of Puget Sound separates the island from the mainland.