Police have closed the A48 in both directions following a road traffic collision involving a motorbike.
Paramedics, firefighters and police officers attended the incident which happened shortly after 5pm between Tutshill and Lydney.
The causalty has been taken to Southmead Hospital in Bristol by ambulance although the extent of his injuries are not known.
A Gloucestershire Police spokesman said officers were treating the incident as serious.
It is believed at this stage only one vehicle was involved.
