Police said the 48-year-old victim was walking near Fifth and Folsom streets around 11 p.m. when a suspect approached and struck him in the head with an unknown object, knocking him to the ground.
A passerby called 911, and the victim was taken to San Francisco General Hospital with life-threatening head injuries, police said.
No details were immediately available about the suspect, who remains at large.
Anyone with information about the attack is encouraged to call the Police Department's anonymous tip line at (415) 575-4444 or send a tip by text message to TIP411.
