The robberies occurred July 12 at 11:25 a.m. at 3701 Balboa St. in the Richmond District, and on Friday at 12:45 p.m. at 5268 Diamond Heights Blvd. in Diamond Heights.
Both crimes were captured on surveillance video.
In each case, a woman described as white, Hispanic or mixed-race approached a teller, brandished a pistol and demanded money, then fled on foot.
"It's definitely the same method," Inspector Phil Wong said. He said a large amount of cash was taken in the second robbery.
The woman is described as in her 30s, 5 feet to 5 feet 3 inches tall and about 130 pounds. She had brown hair tinted red or orange and was wearing sunglasses.
Anyone with information about the suspect is asked to contact Inspector Wong at (415) 553-9208.
A third San Francisco bank was robbed - also by a woman - on Monday morning, but the suspect is not believed to be the same, according to Wong.
In that case, a black woman in her mid-20s robbed a Chase bank at 926 Taraval St. in the Sunset District at about 9:50 a.m.
She did not show a weapon and was wearing eyeglasses and a distinctive black leather jacket with Donald Duck on it, Wong said. She fled with a man believed to be her accomplice.