Reach Island, or Treasure Island, is an island in Case Inlet in the southern part of Puget Sound in the state of Washington, USA. The island's original name was Oak Island, but they later renamed it Reach Island due to its southern neighbor Stretch Island. It forms part of the unincorporated Mason County community of Allyn-Grapeview. The island has a land area of 0.36755 km² (90.82 acres) and a population of 149 persons as of the 2000 census.
U.S. Geological Survey Geographic Names Information System: Reach Island