Rapid is the brand name given to the bus rapid transit system in San Diego County, California. The system serves nearly half the county, operating mainly on the HOV lanes on Interstates 15 and 805, with most of the stops also served by other routes. In addition, there are stations, dubbed as CenterLine in the medians of Interstate 15, Park Boulevard in San Diego and on East Palomar Street in Chula Vista, that are designed in a similar manner to the light rail stations. The system operates with a dedicated fleet, although buses from the Mainline fleet are regularly substituted. The system is administered, built and managed by San Diego Association of Governments (SANDAG) and is operated as part of the San Diego Metropolitan Transit System (MTS).

The San Diego Metropolitan Transit System is a public transit service provider for central, southern, northeast, and southeast San Diego County, California, as well as for the city of San Diego. The agency directly operates a large transit system that includes the MTS Bus, San Diego Trolley light rail, and Rapid bus rapid transit services. The MTS also controls the San Diego and Arizona Eastern (SD&AE) freight railway and regulates taxicabs, jitneys, and other private for-hire passenger transportation services.