Firefighters said it is believed the fire was started by a male patient in one of the rooms at the Longley Centre at 5.20am this morning.
Crews from Elm Lane, Rivelin and Central fire stations were called out to the incident. There are not believed to be any injuries.
Wayne Sutcliffe, watch manager at Elm Lane Fire Station, said: "The fire was started in one of the rooms and it caused damage to some bedding. The smoke also spread out into the Maple Ward.
"Staff had put it out when we arrived with fire extinguishers.
"It is believed it was started deliberately but the exact cause is not known at this time and investigators are at the scene this morning."
Firefighters used a positive pressure ventilation fan to clear the smoke from the facility.
He added that it was not deemed necessary to evacuate people from the building and crews left the scene after about an hour.
The Maple Ward is described as an acute admissions ward which has seventeen beds for mental health patients and a 'Place of Safety' suite.
Patients include those who could be suffering from schizophrenia and depression.