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 Pittsburg/Bay Point and Concord. Tri Delta Transit buses also connect with County Connection bus service, WestCAT bus service, Delta Breeze bus service and Amtrak at shared bus stops. In 2021, the system had a ridership of 905,800, or about 3,400 per weekday as of the second quarter of 2022.