The three-alarm fire started sometime after 7 p.m. in a mechanical room on the third floor of the hospital's subbasement, according to Leslie Hynes with Snohomish County Fire District 1.
The fire sent smoke billowing through the stairwells, but didn't spread to any other areas in the hospital.
No patients were injured, though many were moved to another part of the hospital and 11 were transported to Providence Medical Center in Everett.
A pregnant woman was transported to Swedish hospital in Ballard.
A spokesperson for Bellevue's Overlake Hospital originally said eight patients were taken to Overlake, but she later said the order to move those patients was cancelled and they never arrived.
Everyone else was asked to shelter in place while fire crews worked on the blaze.
Employee Kevin Brunink was inside the building when the fire started.
"The lights just went out, we lost the computers and the generator kicked in about 30 to 40 seconds later. They just started calling fire alarms on the 8th floor, 9th floor, 5th floor," he said.
Fifteen-year old Lacey Tolbert was trapped in a room on the fifth floor with her grandmother and her 12-year old brother when thick, grey smoke began wafting into the room.
"It was thick and it made your eyes burn and your lungs burn, too," Tolbert said.
Firefighters eventually knocked down the fire, and investigators are now looking for the cause.