The San Diego Metropolitan Transit System (SDMTS or often simply MTS) is the public transit service provider for Central, South, Northeast and Southeast San Diego County. 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.

MTS is one of the oldest transit systems in Southern California, with predecessors dating back as early as the 1880s. The current agency started operations in 1976 as the San Diego Metropolitan Transit Development Board (MTDB) and changed to its current name in 2005.

The MTS works closely with the North County Transit District (NCTD), which operates public transit services in Northern San Diego County, and the San Diego Association of Governments (SANDAG), which plans, develops, and constructs transit projects for both the MTS and NCTD.

The average daily ridership among all public transit services provided by MTS was 282,300 in the Fourth Quarter of 2019 and saw an annual ridership of 86,554,400 in 2019.