Two vehicles crashed on the B4421 between Newborough and Llangaffo on Anglesey on Sunday just after 20:00 GMT, North Wales Police said.
A local woman was taken to Ysbyty Gwynedd in Bangor by ambulance.
The road was closed for nearly six hours while the vehicles were recovered. Anyone with information is asked to call 101.
Police have appealed for witnesses following a road collision that left a woman with significant injuries.