The break on 15th Avenue near Ulloa Street was first reported around 1 p.m., Jue said.
Two blocks on 15th Avenue between Ulloa and Taraval streets will remain without water service until the repairs to the 81-year-old cast iron pipe are completed, but other areas are unaffected, according to SFPUC spokesman Tyrone Jue.
The break is disrupting service to Municipal Railway light-rail service, utility and Muni officials said.
Muni officials first issued an alert on Twitter about the water line break at 1:48 p.m.
Outbound light-rail vehicles on the L-Taraval line are stopping at the West Portal station and bus shuttles are taking passengers the rest of the way to the San Francisco Zoo, Muni officials said.
The break occurred only two blocks from the scene of an incident last year that damaged several houses.
A 16-inch cast iron water main broke at 15th Avenue and Wawona Street on Feb. 27, 2013, and flooded 23 homes.
Three homes were red-tagged as a result of that incident and three yellow-tagged, according to reports from shortly after the incident. A dozen cars were also damaged.