Glasgow Street was closed in both directions and nearby residents asked to stay indoors with windows and doors closed as crew tackled the fire in Howgate Street.
The fire was reported to Scottish Fire and Rescue Service (SFRS) at 3.45 am on Monday.
Two pumps from Dumfries initially attended but they were quickly joined by another pump from the town and support from appliances from Thornhill, Lockerbie, Dalbeattie and the control unit from Dreghorn.
A statement from SFRS said: “Crews were faced with a well-developed fire on arrival and they used high pressure hose reels, main jets and a water tower to tackle the blaze in the one storey commercial premises.
“The fire had also spread to an adjacent building.”
It addded: “Firefighters also had to cool acetylene cylinders which were at the premises and they are continuing damping down operations at this time.”
Police Scotland also attended the blaze.
It is not believed that anyone was in the building at the time of the fire.
Building control had been called and SFRS’s fire investigation unit will visit the scene today to establish the cause.