The accident happened at around 3pm on B4235 near the village of Llangwm and involved a single, red and black Yamaha R1 motorcycle.
The rider was taken to the Royal Gwent Hospital in Newport with serious injuries.
The road was closed for approximately four hours while Gwent Police conducted an investigation.
Officers are appealing for members of the public who may have been in the area at the time and have further information, to call 101 quoting log number 302 of May 29.