Fire was reported in a multi-unit building at 550 Leavenworth Street near Geary Street at around 8:30 a.m., San Francisco police spokesman Albie Esparza said.
The fire was controlled in about an hour and no injuries were reported.
Arson investigators were called to the scene and the fire has been deemed suspicious.
The fire department said several of the building's residents have been displaced and were looking for temporary alternative housing.
Further information was not immediately available.