The robbery was reported at about 3:15 a.m. near Third Street and Evans Avenue.
The 76-year-old man was at the gas station and was planning on heading to Chinatown when he was approached by a woman who knocked him down, grabbed a coin purse from him and ran away, according to police.
The victim was taken to San Francisco General Hospital after complaining of pain to the back of his head, police said. His injuries are not considered life-threatening.
The woman, described as in her 40s, had not been found as of this morning.
Anyone with information about the robbery is encouraged to call the Police Department's anonymous tip line at (415) 575-4444 or send a tip by text message to TIP411.