The 66-year-old man was hurt after his Yamaha Tracer 9 bike was in collision with a Fiat Ducato van on the A714 near Bargrennan at 11.55 am on Sunday morning.
He was airlifted from the scene and taken to Queen Elizabeth University Hospital in Glasgow.
Castle Douglas roads policing officers are appealing for witnesses and information and PC Josh Maxwell said: “We’re appealing for anyone who witnessed the incident, or was in the area around that time and may have potential dashcam footage, to please come forward.
“If you believe you can assist our enquiries, please call 101, quoting incident 1488 of 20 March.”
