Four fire engines and 21 firefighters and officers were called to a fire at Rio spa in Kentish Town Road, Kentish Town.
A sauna at the rear of the ground floor was damaged by fire.
The Brigade was called at 1300 and the fire was under control at 1409. Fire crews from Kentish Town, Euston and Hornsey fire stations attended the scene.
The Brigade’s 999 control officers took more than 35 calls to the fire.
The cause of the fire is under investigation.