Foster Island may refer to:
Foster Island is an island in Seattle, Washington, USA's Union Bay. Bisected by State Route 520, it is part of the Washington Park Arboretum. There is a walking trail connecting Foster Island, and the adjacent Marsh Island, to mainland Seattle via a set of bridges and boardwalks.
Coordinates: 47°38′32.5″N 122°17′29.7″W / 47.642361°N 122.291583°W / 47.642361; -122.291583