Officer Carlos Manfredi said witnesses in the city's Bayview District reported hearing the woman screaming. She was taken to a hospital burn unit with life-threatening injuries.
Investigators believe her attacker was 22-year-old Dexter Oliver.
Manfredi said Oliver had a "dating relationship" with the woman, but he did not know the circumstances that led to the attack.
The victim's sister told KGO-TV that Oliver doused his girlfriend with gasoline and lit her on fire during an argument at a nearby Laundromat.
Precious Craig said most of her sister's face was burnt, and that the family expects her to recover.
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