Fort Casey, Military fort and state park in Whidbey Island, United States

Fort Casey is a former military installation and state park on Whidbey Island in Washington, covering about 190 hectares with over three kilometers of shoreline along Puget Sound and Admiralty Inlet. The site includes concrete batteries, underground passages, a restored lighthouse, and open grass areas overlooking the waterway.

Fort Casey was named for Brigadier General Thomas Lincoln Casey, U.S. Army Chief of Engineers, and construction began in 1897 under the Endicott Board's modernization program. With Fort Flagler and Fort Worden it formed a "triangle of fire" guarding Admiralty Inlet. Its disappearing guns became active in 1901 but were obsolete almost at once; most were removed in World War I for railcar mounting, and the rest were scrapped in World War II.

The name Casey honors Thomas Lincoln Casey, a US Army engineer from the late 19th century. Visitors today see massive concrete bunkers and gun emplacements set into the coastal landscape, surrounded by grassy fields.

The park requires a ten-dollar day pass or a valid Discover Pass for entry. Walking trails lead through the grounds, and the shoreline is suitable for diving and beach activities.

The two 10-inch disappearing guns at Battery Worth are M1895MI pieces No. 26 and No. 28 on M1901 carriages, originally mounted at Battery Warwick, Fort Wint, on Grande Island in the Philippines. They came to Fort Casey in the 1960s together with two 3-inch M1903 guns at Battery Trevor, which had served at Battery Flake, Fort Wint.