Allyn is a census-designated place in Mason County, Washington, United States. The population was 1,963 at the 2010 census. It was part of the former Allyn-Grapeview CDP that was broken up into Allyn and Grapeview in 2010. Allyn is located on the western shore of North Bay region of the Case Inlet of the Puget Sound. The area's main road and thoroughfare is State Route 3.