Fort Mason, Military fort in Marina District, San Francisco, United States

Fort Mason is a former US Army post on a headland in San Francisco's Marina District, split into Upper Fort Mason, site of the original coastal fortifications, and Lower Fort Mason, the former military port with its piers and warehouses. The historic district covers 1,200 acres (490 ha) and holds 49 buildings of historic significance. A two-story white clapboard building with a steep green shingle roof, four-column portico and red brick steps stands among them.

The installation served as the San Francisco Port of Embarkation during World War II, coordinating the dispatch of more than 1.6 million troops to Pacific theaters. After decommissioning in the 1970s, the grounds transformed into a space for arts and community activities.

Fort Mason Center occupies the former military port's piers and warehouses, now used for cultural and non-profit activities. The site holds over 30 non-profit organizations, three museums and two colleges, and hosts more than 15,000 events a year. The National Park Service offices for the Golden Gate National Recreation Area and the San Francisco Maritime National Historical Park are also here.

The grounds remain open daily, with several restaurants, cafes, and art galleries accessible to walk-in visitors. On clear days, views across the bay reach Alcatraz Island and the Golden Gate Bridge.

The Interval cafe houses a mechanical clock engineered to run for 10,000 years, part of a project for long-term technological preservation. Designed by the Long Now Foundation, the clock operates on weights and springs without electrical power.