The Market Street subway is a two-level subway tunnel that carries Muni Metro and BART trains under Market Street in San Francisco, California. It runs under the length of Market Street between Embarcadero station and Castro station. The upper level is used by Muni Metro lines and the lower level is used by BART lines. BART does not run through the whole subway; it turns south and runs under Mission Street southwest of Civic Center/UN Plaza station. The northeastern end of the BART level is connected to the Transbay Tube. On the Muni Metro level, the southwestern end of the Market Street subway connects to the much-older Twin Peaks Tunnel, and the northeastern end connects to surface tracks along the Embarcadero.

Muni Metro is a semi-metro system serving San Francisco, California, United States. Operated by the San Francisco Municipal Railway (Muni), a part of the San Francisco Municipal Transportation Agency (SFMTA), Muni's light rail lines saw an average of 75,500 boardings per day as of the fourth quarter of 2023 and a total of 24,324,600 boardings in 2023, making it the sixth-busiest light rail system in the United States.