The San Francisco – Oakland Bay Bridge (known locally as the Bay Bridge) is a pair of bridges spanning San Francisco Bay of California, in the United States. As part of Interstate 80 and the direct road route between San Francisco and Oakland, it carries approximately 270,000 vehicles per day on its two decks. It has one of the longest spans in the world.

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, 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 main spans of roughly equal length, a western span connecting downtown San Francisco to Yerba Buena Island and an eastern span connecting the island to Oakland. The main part of the western span is a suspension bridge while the main part of the eastern span is a cantilever bridge. During the 1989 Loma Prieta earthquake, a section of the eastern span's upper deck collapsed onto the lower deck and the bridge was closed for a month. Reconstruction of the eastern span as a self-anchored suspension bridge began in 2002 and is scheduled to open in 2013.