Crews responded to the scene, in the 1200 block of Mill Creek Boulevard, just after 2 a.m. after receiving a report of a blaze, said Autumn Waite of Snohomish County Fire District 7.
Smoke was visible from first-floor apartment when firefighters arrived, and the building already had been evacuated. There were no injuries.
After the flames were extinguished, investigators found fire damage to one unit and smoke damage to two others.
The cause of the fire remains under investigation.