The incident, around 11.30 am on Tuesday, happened at the suspension-style pedestrian crossing of the busy trunk road at Cuckoo Bridge.
Police Scotland say the 15-year-old was treated at the scene before being taken to Dumfries and Galloway Royal Infirmary with serious injuries.
Inspector Stephen Stiff, who is leading the enquiry, explained that the teenager was fortunate not to be struck by any vehicles.
He said: “We are appealing for anyone who may have been in the area to get in touch if they saw this incident.”
Anyone who can help is asked to contact the police general telephone number 101.
NEWSDESK: 01461-202417
