Bay Area Rapid Transit (BART) is a rapid transit system serving the San Francisco Bay Area. The heavy-rail public transit and subway system connects San Francisco with cities in the East Bay and suburbs in northern San Mateo County. BART operates five lines on 104 miles (167 km) of track with 44 stations in four counties. With an average weekday ridership of 367,591 passengers, BART is the fifth-busiest heavy rail rapid transit system in the United States.

BART is operated by the San Francisco Bay Area Rapid Transit District, a special-purpose transit district that was formed in 1957 to cover San Francisco, Alameda County, and Contra Costa County. The name BART is pronounced as a word, not as individual letters. In some ways, BART is the successor to the Key System, which ran streetcars throughout the East Bay and across the lower deck of the San Francisco – Oakland Bay Bridge until 1958.

BART has served as a rapid transit and commuter rail system, and provided an alternative transportation route to highway transportation, though its critics counter its success has taken four decades to come to fruition at a steep cost during the interim. The system is being expanded to San Jose with the Silicon Valley BART extension.