The fire broke out at 7:10 a.m., firefighters said.
District Fire Chief Gerald Hogan said all 12 residents were safely out of the building in the community of Allston when crews arrived.
Firefighters rotated while fighting the blaze to stave off heat exhaustion, the Boston Herald said.
The heaviest concentration of fire was on the first and second floors in the rear of the structure, Hogan said, and the inferno melted the siding off the three abutting residences.
The cause of the fire is under investigation.