Muni lines 26, 27 and 67 were rerouted at about 7:15 a.m., spokeswoman Kristen Holland said.
The break occurred at the intersection of 26th and Valencia streets at about 4 a.m., according to San Francisco Public Utilities Commission spokesman Tony Winnicker.
It was a break on an 80-year-old, 16-inch main on 26th Street. Once it was determined where the break was officials shut down that section of the main at about 7 a.m., he said.
Northbound Valencia Street reopened at 7:15 a.m. Southbound Valencia from 25th Street to Cesar Chavez will remain closed until this afternoon, but it should be reopened for the evening commute, he said.
Water pressure that slowed down or stopped was restored to the area by about 7:15 a.m., according to Winnicker.
The exact cause of the break is unknown, but there tend to be more breaks during the winter, he said.