The break was reported just before noon in a 4-inch cast-iron water main near Stockton and Sutter streets, San Francisco Public Utilities Commission spokeswoman Suzanne Gautier said.
The repairs kept the tunnel closed until around 8 p.m. this evening, officials said.
No water service in the area was interrupted, Gautier said.
The 30-Stockton and 45-Union bus lines were rerouted around the closed tunnel during repairs, Muni spokesman Paul Rose said.
Gautier said the break was due to the old age of the pipe, which dates to 1914, and not connected with construction on Muni's Central Subway project.
A boring machine is operating about 100 feet north of where the leak occurred. The machine is being used underground to carve out a tunnel for the new subway line that will eventually connect passengers from the San Francisco Caltrain Station to Chinatown, according to Rose.