The San Francisco–Oakland Bay Bridge (known locally as the Bay Bridge) is a complex of bridges spanning San Francisco Bay of the U.S. state of California. As part of Interstate 80 and the direct road route between San Francisco and Oakland, it carries approximately 240,000 vehicles per day on its two decks. It has one of the longest spans in the United States.

The toll bridge was conceived as early as the gold rush days, but construction did not begin until 1933. Designed by Charles H. Purcell, and built by American Bridge Company, it opened for traffic on November 12, 1936, six months before the Golden Gate Bridge. It originally carried automobile traffic on its upper deck, and trucks and trains on the lower, but after the closure of the Key System transit lines, the lower deck was converted to road traffic as well. In 1986, the bridge was unofficially dedicated to James B. Rolph.

The bridge consists of two sections of roughly equal length; the older western section connects downtown San Francisco to Yerba Buena Island and the newer eastern section connects the island to Oakland. The western section is a double suspension bridge. Originally, the largest span of the original eastern half was a cantilever bridge. During the 1989 Loma Prieta earthquake, a section of the eastern section's upper deck collapsed onto the lower deck and the bridge was closed for a month. Reconstruction of the eastern section of the bridge as a causeway connected to a self-anchored suspension bridge began in 2002; the new bridge opened September 2, 2013 at a reported cost of over $6.5 billion and is currently the world's widest bridge, according to Guinness World Records.