The victim, believed to be in his 50s or 60s, was found around 12:15 a.m. in an alcove in front of a building in the 900 block of Columbus Avenue, between Chestnut and Lombard streets, police spokesman Officer Gordon Shyy said.
The man had suffered head trauma and was taken to San Francisco General Hospital. His injuries are considered life-threatening, Shyy said.
No arrests have been made in the attack, he said.
Anyone with information about the case is asked to call the Police Department's anonymous tip line at (415) 575-4444 or send a tip by text message to TIP411.