A young person playing with a lighter caused Monday's fire at the Explorer Hotel in Yellowknife.
City officials say a minor lit a book on fire in a room on the second floor, activating the hotel's sprinkler system.
Fire crews arrived to find heavy smoke in the second floor room, but the sprinkler system had put out most of the fire. Firefighters extinguished the remaining small fire.
No one was hurt in the fire, but there is extensive water damage to the hotel's second floor and lobby.
Guests were initially evacuated from the hotel during the fire, but were sheltered in the hotel's conference rooms once it had been put out.
After the sprinklers stopped flowing, they were allowed to return to their rooms using the stairwell.
Guests on the second floor have been moved to rooms on other floors.