The eastbound carriageway of the A27 at Falmer was closed shortly after 8.30pm on Wednesday (September 25) when the Constable from the Surrey and Sussex Road Policing Unit was struck and injured by a vehicle and taken to hospital.
A police spokesman said the collision is not being treated as a deliberate act.
According to police, the officer sustained serious but not life threatening injuries and his next of kin have been informed.
He had been helping at the scene of a broken down lorry on one of the lanes of the carriageway since 7.35pm when he was struck by a passing vehicle just as the lorry was about to move again.
The carriageway was closed for the scene to be examined and made safe, and drivers, including those leaving after the Brighton and Hove Albion cup game with Aston Villa from the nearby Amex Stadium, were warned of possible delays and asked to avoid the area if possible.
A crash investigation is underway and the driver of the vehicle involved in the collision, who has not been arrested, is co-operating with the investigators, police said.
Anyone who saw what happened is asked to contact Sussex Police online or by calling 101, quoting Operation Fenwick.
This is the third police officer to have been seriously injured while responding to incidents this week.
On Monday (September 23), at approximately 1.05am, two police officers and a man they had detained were hit by a car on the A259 at Littlehampton.
The policeman remains in hospital and is due to have surgery on a broken shoulder, an arm and leg. His colleague, a policewoman, has been discharged from hospital after suffering two fractures and extensive bruising, and is recovering at home.
A local man, who the officers arrested for possession with intent to supply Class B drugs moments before the impact, also suffered fractures but has left hospital and been released under investigation.
Jack Robson, 20, unemployed and from Holly Drive, Wick, Littlehampton, appeared at Crawley Magistrates’ Court on Wednesday (September 25) charged with three counts of causing serious injury by driving dangerously, burglary, driving with no insurance, not in accordance with his provisional licence and with uncorrected vision.