The A470 has reopened after it was closed for more than three hours.
Police said a man jumped from a moving white van on the southbound carriageway at Taffs Well at approximately 3.55pm.
The man was taken to the University Hospital of Wales with serious injuries where his condition is described as critical.
The northbound carriageway was also closed for a period of time in order for a helicopter to land.
Stagecoach Wales said it experience delays to its services due to the volume of traffic.
Anyone with information is asked to call 101.