Motorists faced delays and diversions following the incident at around 3pm on Thursday on a northbound stretch of the road at Felton.
Northumbria Police said one woman was taken to hospital with serious injuries although her current condition is unknown.
Highways England confirmed shortly before 10pm that the A1 had reopened.
The police earlier said: “Shortly after 3pm today (Thursday), officers received a report of a collision involving a car and a tractor on the A1 northbound at Felton.
“Emergency services are currently at the scene. A female occupant of the car has been taken to hospital with serious injuries.
“A clean-up operation is ongoing and the A1 is currently closed in both directions. The road is expected to remain closed for the next few hours and a diversion is in place.
“Drivers heading northbound are asked to exit the A1 at Causey Park and follow the A1068 towards Amble and Alnmouth. The diversion will then take drivers on the B1340 towards Bamburgh for one mile, before re-joining the A1 north of Alnwick.
“Motorists travelling southbound are asked to leave at the A1068 at Alnwick and follow the signs, before re-joining the A1 at Causey Park.
“Anyone with information regarding the collision is asked to contact 101 quoting reference 662 120919 or Crimestoppers anonymously on 0800 555 111.”