Gloucestershire police were called at 4.45pm with a report of a collision involving a motorcyclist and car near Coaley Peak viewpoint.
A man was taken to Southmead Hospital in Bristol with serious injuries.
Frocester Hill in Nympsfield was closed at the B4066 crossroads junction with Bath Road at around 5.30pm and was eventually reopened.
A spokesperson for the force said: “Road closures have been lifted following the serious collision on Frocester Hill.
“A man has been taken to Southmead Hospital in Bristol with serious injuries and his next of kin is aware.
“Officers would like to thank motorists for their patience and co-operation during the closure.”
Gloucestershire police are asking anyone who witnessed the collision and couldn’t stop or who has dashcam footage to get in contact.
Information can be provided to police online by completing the following form and quoting incident 374 of May 11, here – https://tinyurl.com/4abtem4u