21 firefighters were called to a private hospital in Lambeth Road on Tuesday night when a fire broke out on the first floor.
Cygnet Churchill – formerly Cambian Churchill – is a private mental health rehabilitation unit located opposite the Imperial War Museum.
Part of a bedroom on the first floor was damaged by fire.
54 patients and 15 staff left the building before firefighters arrived.
There were no reports of any injuries.
The brigade was called at 9.40pm and the fire was under control just over 40 minutes later.
Fire crews from Lambeth and Soho fire stations attended the scene.
The cause of the fire is under investigation by the brigade and the Metropolitan Police Service.