Police received reports of a collision between a car and a cyclist at Hop Oast Roundabout, Worthing Road, Horsham, at around 2.20pm on Tuesday, August 20.
The cyclist, a 49-year-old man from Brighton, suffered serious injuries and was taken to the Royal Sussex County Hospital, Brighton.
The driver of the Fiat 500, a 71-year-old man from Worthing, was shaken but uninjured in the collision.
The A24 was closed just before 3pm from Southwater roundabout, with traffic being redirected through Southwater. The road is expected to remain closed until at least 6pm, with traffic disruption expected between Buckbarn to Southwater.
Police are appealing to witnesses of the collision to contact Sussex Police on 101, quoting serial 889 of 20/0