The blaze was reported about 10:30 p.m. in the 23700 block of 101st Avenue Southeast after a maintenance worker heard a smoke alarm going off on the third floor of the complex.
He knocked on the door but nobody answered, so the worker entered the apartment and found thick black smoke. The maintenance worker called 911.
Firefighters managed to contain the fire to a single unit, which was heavily damaged, but the apartment underneath the unit suffered water damage.
No one was injured.
A cause has not been determined.