The fire was reported just after 5 a.m. at 433 Sonoma Ave., according to Santa Rosa fire Battalion Chief Mark Basque.
Firefighters arrived within four minutes and found a first floor apartment of the two-story, 18-unit complex fully involved, Basque said.
All residents of the building safely evacuated and were not injured, according to Basque.
Firefighters reportedly had the flames controlled within 20 minutes.
The residents of the apartment that burned were not home at the time of the fire. The unit and its contents were destroyed, Basque said. The official cause remains under investigation.