The fire was reported at 234 Seventh Ave. at 3:10 a.m. and controlled in about 10 minutes, Battalion Chief Mike Borean said.
One resident was still asleep when firefighters arrived, so they woke him and asked him to evacuate, Borean said.
A preliminary investigation revealed the fire likely started when old wiring in an electrical outlet in one of the building's four apartment units caused a spark, Borean said.
The fire displaced nine residents for the night, but all but two should be able to return home once the building has aired out, Borean said. The couple living in the unit where the fire started will have wait longer to return.
No injuries were reported.