The Key System (or Key Route) was a privately owned company which provided mass transit in the cities of Oakland, Berkeley, Alameda , Emeryville, Piedmont, San Leandro, Richmond, Albany and El Cerrito in the eastern San Francisco Bay Area from 1903 until 1960 when the system was sold to a newly formed public agency, AC Transit.

The Key System consisted of local streetcar and bus lines operating solely in the East Bay, and a network of commuter rail and bus lines connecting cities and neighborhoods in the East Bay to San Francisco by way of a ferry pier extending out into San Francisco Bay, and later, via the lower deck of the Bay Bridge. At its height during the 1940s the Key System had over 66 miles of track that connected the communities of Richmond, Albany, Berkeley, Oakland, and San Leandro with each other and to San Francisco.

The local streetcars were discontinued in 1948 and the commuter trains to San Francisco were discontinued in 1958. The Key System's original territory is today served by BART and AC Transit bus service.