The fire, reported at 7:07 p.m. started in the bathroom then burned through the door into the bedroom, causing the sprinkler head to activate along with the fire alarm at the Riverwood Apartments, 24600 block of Russell RoadSouth.
Puget Sound Fire units arrived with smoke coming from the front door. Entry into the home was quickly made, and the fire extinguished in the bathroom.
Thanks to the sprinkler in the bedroom, the fire did not get beyond the bathroom door, Puget Sound Fire said.
The apartment below the fire had damage from the water used to extinguish the fire above.
American Red Cross was called and arrived to help the two displaced families.
The cause of fire is still under investigation.