Hat Island is a census-designated place (CDP) in Snohomish County, Washington, United States. The population was 41 at the 2010 census. The CDP occupies an island in Possession Sound called Hat Island, also known as Gedney Island. The island lies in Possession Sound amongst the mainland city of Everett, Washington, and the southern part of Whidbey Island. Gedney Island has a land area of 1.768 km (436.8 acres) and a population of 13 people was reported as of the 2000 census.

Gedney Island was first mentioned in the historical book by naturalist Archibald Menzies of the Vancouver Expedition in 1792. It was named by Charles Wilkes in 1841; although other sources say it was for John B. Gedney or Jonathon Haight Gedney, Wilkes’s memoirs say he named it for Lt Thomas R. Gedney. The name Hat is for the involve of a seashore and treeless ledge, first used in 1870. Since 1980 the island would be called Gedney Island (Hat).

There is currently one walk-on ferry named the Hat Express which transports walk-on passengers to and from the island. The maximum capacity of the ferry is 88 passengers. It runs a limited schedule upon Thursdays, Friday evenings, and weekends in the middle of the Everett Yacht Club on the west subside of 13th Street in Everett, and the Hat Island Marina.

According to the United States Census Bureau, the CDP has a total area of 0.692 square miles (1.79 km), of which, 0.688 square miles (1.78 km²) of it is house and 0.004 square miles (0.01 km) of it (0.58%) is water.