The robbery was reported at about 10:30 p.m. at the plaza, located on Market Street between Seventh and Eighth streets.
The 44-year-old victim was approached by a man who asked for a cigarette, according to police.
As the victim was pulling out a cigarette, the suspect began to punch and kick him repeatedly, knocking him unconscious, police said.
The suspect then sifted through the victim's pockets, taking cash before fleeing. He had not been found as of this morning, police said.
The victim was taken to San Francisco General Hospital to be treated for his injuries. He is expected to survive.
Anyone with information about the case is encouraged to c all the Police Department's anonymous tip line at (415) 575-4444 or send a tip by text message to TIP411.