Emergency services were called to Ninewells Hospital on Friday evening after reports of a fire in a toilet block.
The Scottish Fire and Rescue Service were called to the scene at 4:45pm and sent six appliances to the hospital.
It is understood that there are no casualties.
One eyewitness said they had been waiting outside for two hours while firefighters dealt with the blaze.
Jamie Jennings who was heading to the hospital to visit a relative said: “There were queues of people waiting to get in and staff waiting to start their night shift.
“There is no smoke that we can see from outside the main reception but they had a hose all the way into the hospital.”
Appliances attended from Blackness Road, Kingsway East, Macalpine Road, Tayport and Balmossie fire stations.
Two appliances remained on the scene almost three hours later and four breathing apparatus, three thermal imaging cameras and two positive pressure ventilation fans were used.
A spokesperson for the Scottish Fire and Rescue Service said: “We were alerted at 4.45pm on Friday, June 26 to reports of a fire within Ninewells Hospital, Dundee.
“Operations Control mobilised a total of six appliances to the scene and firefighters assisted in evacuating the premises. There are no casualties.
“SFRS crews are presently working to extinguish the fire which has taken hold within a toilet block of the building.”
A spokeswoman for NHS Tayside said: “A small fire broke out in the outpatient area on level seven of Ninewells Hospital late this afternoon.
“Scottish Fire and Rescue Service attended the hospital and the area was evacuated.
“No patients were in the vicinity and there was no disruption to services.”