The fire was knocked down at 4:27 a.m., according to Alameda fire Captain Jim Colburn.
Fire officials received a report of the fire at 1203 Park St., near San Jose Avenue, at 2:37 a.m., Colburn said.
Firefighters arrived at the three-story building, located near Jackson Park, within minutes to find an active fire on the building's top two floors.
The fire did not spread to the two commercial units on the ground floor, a clothing store and a nail salon.
Colburn said one of the businesses below the apartments may have water damage.
The fire was contained to one building and did not impact surrounding structures.
Firefighters are still on scene and working to extinguish hidden hotspots in the building as of about 6:30 a.m., according to Colburn.
At least six residents in the six apartments have been displaced. The Red Cross responded to the scene to assist displaced residents, Colburn said.
No injuries have been reported and the cause of the fire has not been determined, according to Colburn.