And police are appealing for information about what happened to the man.
He was discovered at Dunreggan Brae, at its junction with the A720, near Moniaive at about 7.25 pm.
Emergency services attended but the 57-year-old was pronounced dead at the scene. He has not yet been named but his bike has been described as a red, Specialized model.
Commenting, Sergeant William Broatch, of the road policing unit in Lockerbie, said: “Our thoughts are with the family and friends of the man who died at this difficult time.
“Our enquiries are ongoing into this incident and we are seeking the public’s assistance to establish exactly what happened.
“If you were in the area at the time and have not yet spoken to police, I would ask you to please contact us as soon as possible. If you were driving and have access to a dash-cam, I would ask you to please check back and see if there is any footage of a man on a bike matching that description that may be able to assist our investigation.”
Anyone with information can contact officers on 101, quoting reference number 3342 of 21 September.
