Crews saw the discharge coming from a manhole near Great Highway and Balboa Street at about 1 a.m., San Francisco Public Utilities Commission spokeswoman Jean Walsh said.
SFPUC and Department of Public Works crews are at the scene doing cleanup this morning, Walsh said.
Walsh said 90 percent of the discharge is rainwater, and that there have been no reports of a foul odor.
She said the discharge is on the sand of the beach and on its pedestrian promenade, and that the spill spans about a block.
The area has been cordoned off and closed to pedestrians, Walsh said. No streets have been closed.
There is no estimate for when that area will reopen, she said.