The building at M Street and 22nd Street once housed the Merced County High School and later the Merced County Library but had been empty for decades.
The Merced Fire Department said they have asked for assistance from multiple fire agencies from Merced and Madera counties to battle the fire.
If possible, people are asked to avoid the area and expect traffic delays and road closures.
Stay with ABC30 for the latest details on this developing story.