Motorists travelling north on the A23 between Warninglid and Handcross were asked to avoid the area for eight hours due to a serious collision on Sunday
At 5.26am on Sunday (28 July) a man riding a motorcycle sustained serious injuries after colliding with the central barrier at Handcross Hill, police said.
He has been taken to the Royal Sussex County Hospital, Brighton.
The A23 northbound was closed while the incident was dealt with and investigated, and reopened just before 1.30pm.
In a statement, Sussex Police said: "The A23 northbound at Handcross re-opened at 1.20pm on Sunday (July 28), some eight hours after being closed while emergency services dealt with a serious collision involving a motorcyclist.
"We would like to thank motorists whose journeys were disrupted for their patience and understanding."
Police have appealed for witnesses to the collision or anyone who saw the motorcycle being driven prior to the 5.26am incident or any dash-cam footage remains.
Details can be passed online, by emailing email@example.com or by phoning 101, quoting Operation Carisbrook.
Sgt Richard Hobbs, acting sergeant on the Roads Policing Unit at Sussex Police, said the collision was serious before confirming the road had been reopened.