Emergency services were called to the scene at Cuckoo Bridge in Dumfries at around 11.30am today.
The 15-year-old boy was not struck by any vehicles on the road but suffered serious injuries. He was taken to Dumfries and Galloway Royal Infirmary for treatment.
Police appealed for information about the incident.
Inspector Stephen Stiff, of Police Scotland, said: “We are appealing for anyone who may have been in the area to get in touch if they saw this incident.
“The boy was not struck by any vehicles on the roadway and appears to have fallen around 30ft onto the road from the bridge.
“Anyone wishing to help can call Police Scotland in Dumfries on 101.”