Possession Sound is part of Puget Sound, located in the U.S. state of Washington between Whidbey Island and the shoreline of Snohomish County approximately between the cities of Everett and Mukilteo. Possession Sound connects the main Puget Sound basin to the south with Saratoga Passage and Port Susan to the north. The Snohomish River flows into Possession Sound at Port Gardner Bay. Gedney Island, also called Hat Island, is located in Possession Sound. The Washington State Route 525, Mukilteo-Clinton passenger ferry run crosses the narrowest section of Possession Sound.