The San Francisco Municipal Railway ( MEW-nee; SF Muni or Muni), is the primary public transit system within San Francisco, California. It operates a system of bus routes, the Muni Metro light rail system, three historic cable car lines, and two historic streetcar lines. Previously an independent agency, the San Francisco Municipal Railway merged with two other agencies in 1999 to become the San Francisco Municipal Transportation Agency (SFMTA). In 2018, Muni served 46.7 square miles (121 km2) with an operating budget of about $1.2 billion. Muni is the seventh-highest-ridership transit system in the United States, with 142,168,200 rides in 2023, and the second-highest in California after the Los Angeles County Metropolitan Transportation Authority.

San Francisco, officially the City and County of San Francisco, is a commercial, financial, and cultural center in Northern California. With a population of 808,437 residents as of 2022, San Francisco is the fourth most populous city in the U.S. state of California. The city covers a land area of 46.9 square miles at the end of the San Francisco Peninsula, making it the second-most densely populated large U.S. city after New York City and the fifth-most densely populated U.S. county, behind only four New York City boroughs. Among the 92 U.S. cities proper with over 250,000 residents, San Francisco is ranked first by per capita income and sixth by aggregate income as of 2022.