The incident involving a Triumph motorcycle and a Land Rover happened on the A465 near Llangua at about 14.45 BST on Saturday.
Gwent Police said the woman was was seriously injured in the collision on the route between Abergavenny and Hereford.
The road was closed for investigation work but has since reopened.
A woman motorcyclist has been airlifted to hospital after a crash with a 4x4 in Monmouthshire.