Case Inlet, in southern Puget Sound in the U.S. state of Washington, is an arm of water between Key Peninsula to the east and Harstine Island to the west. Its northern end, called North Bay, reaches nearly to Hood Canal, creating the defining isthmus of Kitsap Peninsula. Case Inlet is the boundary between Pierce County and Mason County. The southern end of Case Inlet connects to Nisqually Reach, part of the southern basin of Puget Sound. Herron Island lies in Case Inlet.