The robbery was reported at about 4:30 p.m. at the mall, located at 865 Market St.
The victim went into the bathroom and was followed by a female suspect who pushed her from behind, causing her to hit her head on a wall and lose consciousness, police said.
The suspect then took the girl's phone and fled. She had not been found as of this morning.
The victim was taken to San Francisco General Hospital to be treated for facial and head trauma, police said. Her injuries are not considered life-threatening.
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 with "SFPD" in the message.