The biker was riding a yellow Suzuki when he was involved in the crash on Scotch Spring Lane near the village of Stainton at 2.45pm on Saturday.
No other vehicles were involved.
"Yesterday (26 March) at 2:48pm officers attended reports of a road traffic collision on Scotch Spring Lane involving a yellow Suzuki motorcycle.
"The rider, a 39 year-old man suffered serious injuries and was taken to hospital by ambulance, where he remains in a critical condition.
"Officers are keen to understand the circumstances of the collision, and would like to hear from witnesses, and those who may have dash-cam footage of the rider prior to the incident.
"If you can help please call 101 quoting incident number 562 of 26 March 2022.