Firefighters say the fire was reported just after 7 p.m. in the 1600 block of East Yamhill Road near South Federal Way.
The fire department says when crews arrived they found flames in a hopper inside the mill and quickly extinguished the flames.
Firefighters say it appeared that some leftover sawdust in the hopper was burning inside the 6' x 6' hopper.
Welders have been working in the area using cutting torches to dismantle the old sawmill, and firefighters believe either heat or a spark from the equipment ignited the sawdust.
No injuries were reported.