The robbery was reported at about 11:20 p.m. near the plaza at Clay and Kearny streets.
A dark-colored, four-door sedan drove up to the 42-year-old victim and two men got out of the car, according to police. One picked up the flowerpot and struck the woman with it, knocking her to the ground.
The suspects then took the woman's cellphone and fled. They were both believed to be in their 20s, and had not been found as of this morning, according to police.
The victim was taken to a hospital to be treated for a minor head injury, police said.
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.