Dyfed-Powys Police attended the incident on the B4358 between Newbridge on Wye and Beulah at about 15:15 BST on Sunday.
The motorcyclist was airlifted to hospital and is in a serious but stable condition.
Police said the driver of the van was travelling towards Beulah.
The driver of a white van is being sought by police after a crash in Powys left a motorcyclist with "possibly life-changing injuries".