Tri Delta Transit, formally the Eastern Contra Costa Transit Authority, is a joint powers agency of the governments of Pittsburg, Antioch, Oakley, Brentwood, and Contra Costa County that provides bus service for the eastern area of Contra Costa County, California, United States. Tri Delta Transit local buses connect to the BART rapid transit system at Antioch, Pittsburg Center, Pittsburg/Bay Point and Concord. Tri Delta Transit buses also connect with County Connection bus service. In, the system had a ridership of , or about per weekday as of. Tri Delta Transit service began on June 6, 1977. Initial service was two local routes in Antioch and Pittsburg, which connected with express bus service serving BART. AC Transit operated the service under contract until 1984, when it was transferred to a private operator.

Overview of fixed-route service

Tri Delta Transit operates in eastern Contra Costa County and has stops in Antioch, Oakley, Brentwood, Bay Point, Pittsburg, Martinez, and Concord. Tri Delta Transit operates local and express routes on weekdays and local routes on weekends/holidays. Major transfer points for the majority of Tri Delta Transit's routes include:

Route list

Tri Delta Transit operates seventeen bus routes. On weekdays, all of them are operated throughout the cities of Concord, Bay Point, Pittsburg, Antioch, Oakley, Brentwood, and Martinez. On weekends, six of them are operated throughout the aforementioned cities except Concord and Martinez. They're intended to be the longer routes to cover the many segments of the weekday-only bus routes. Even though most of the routes have the same terminus, various of them have their own ways of reaching their terminus.