Traffic was diverted through Annan and the road at the scene was closed for more than six hours while a full examination was carried out.
A police spokesman said: “This afternoon numerous 999 calls were received from passing motorists about a female standing on the outside edge of the safety barrier on the B722 Annan to Eaglesfield road, at the bridge which crosses the A75 near to Northfield, Annan.
“Officers attended and both roads were closed. Local diversions were put in place while the incident was ongoing.
“About 2.50 pm the female fell from the over bridge onto the A75 roadway. She was treated by ambulance staff at the scene and conveyed by air ambulance to Queen Elizabeth University Hospital in Glasgow where her injuries are described as serious.”