High Quality Content by WIKIPEDIA articles! Harstine Island (also known simply as Harstine or Harstene) is an island in Mason County, Washington, United States. The US Census recognizes it as an unincorporated community.The island is located west of Case Inlet in southern Puget Sound, 16 km (9.9 mi) north of Olympia. It has a land area of 48.305 km2 (18.651 sq mi), and had a population of 1,002 as of the 2000 census.