The stabbing was reported at 2:17 a.m. in the 700 block of Geary Street, near Leavenworth Street.
The victim was found stabbed multiple times in the chest and abdomen, police said. He was taken to San Francisco General Hospital to be treated for his injuries, which are considered life-threatening.
The suspect, described only as a man in his 20s, had not been found as of this morning, according to police.
Anyone with information about the stabbing is asked to call the Police Department's anonymous tip line at (415) 575-4444 or to send a tip by text message to TIP411.