No one was injured in the blaze that broke out on the ninth floor of the Chalerm Phrakiart Building at 6:30 am.
Six fire engines took about an hour to put out the fire.
Pol Lt-Gen Jongjet Ao-jenpong, the surgeon-general of the hospital, said the fire damaged only the ninth floor.
He said some patients were evacuated during the fire fighting and were returned to their rooms after the blaze was put out.
The medical services on the ground floor continued as usual while fire fighters were controlling the blaze, Jongjet said.
Police Commissioner-General Pol-Gen Adul Saensingkaew, who rushed to the scene, said the fire was apparently caused by a short circuit inside the Sant Sarutanon auditorium, which is under renovation. He said the soundproof wall cushion caused the fire to spread fast and added that the automatic fire extinguishing system was not operating when the fire broke out.
Adul also complimented Pol Lt Col Withoon Luang-amornsak, deputy Crime Suppression Sub-division 3, for leading eight officers to fight the fire before the fire engines arrived.