All three emergency services attended the scene on the A45 westbound between Great Doddington and Earls Barton.
A spokeswoman for Northamptonshire police said: “Police were called at 6.50am to report a single car which was off road.
“The incident took place on the stretch of road between Great Doddington and Earls Barton, on the westbound carriageway.
“A female motorist was trapped in the vehicle. Police, fire and ambulance attended the scene and the fire service had to cut her out of the vehicle.
“The motorist was taken to Northampton General Hospital with serious but not life threatening injuries.
“An investigation has been launched into why the car was off road, that will be ongoing.”
It is understood that the motorist was out of the car by 8.30am.