Reach Island, or Treasure Island, is an island in Case Inlet in the southern part of Puget Sound in the state of Washington, United States. The island's original name was Oak Island, but was 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 119 persons, as of the 2010 census.