Fort Barry, Military fort in Marin Headlands, United States.

Fort Barry is a military installation in the Marin Headlands with multiple artillery batteries, including Battery Mendell equipped with two large breech-loading guns on disappearing gun carriages. The structures and fortifications spread across hilly terrain overlooking San Francisco Bay.

The installation was established in 1908 to protect the entrance to San Francisco Bay and served as part of the coastal defense system throughout the 20th century. It played a key role in guarding against threats from the ocean.

Batteries at Fort Barry carry the names of officers: George Mendell, Barton S. Alexander, Edwin Guthrie, Samuel Rathbone and Patrick O'Rorke. Battery Construction No. 129, begun in 1943 for two 16-inch guns, was never armed or named. The ground above it later served as the radar and control area for Nike Missile Site SF-87, whose launch area lay in Fort Cronkhite.

A half-mile (0.8 km) tunnel links Fort Barry with Fort Baker; in June 1937 its width was enlarged to 20 feet (6 m). The site lies in the Marin Headlands north of San Francisco, and the surviving batteries, balloon hangar and Battery Construction No. 129 on the summit can be reached on foot.

The balloon hangar at Fort Barry was completed on 27 June 1921 and abandoned the same year, after the Army's brief trial of tethered balloons for coastal defense. The 24th Balloon Company moved its balloon inside shortly after completion. It is the only surviving hangar of its type that actually housed an Army balloon, and one of only two known to survive in the country.