More than 200 firefighters battled the blaze at Bochum's Bergmannsheil hospital.
Police said in a statement that the fire appeared to have started in a patient's room on the 6th floor at about 2:30 a.m. (0030 GMT) Friday, according to AP.
The Bergmannsheil hospital was built in 1888 to treat workers injured in the coal mines of the surrounding Ruhr area. It is considered one of the world's first specialist accident and emergency clinics.