Ronald Barret, 74, of Lancaster was riding a motorcycle which crashed on the A712 just north of Glenamour Farm at about 3.30 pm.
He was flown from the scene to the Queen Elizabeth University Hospital in Glasgow by Helimed but died there on Monday morning.
Police enquiries into the cause of the crash are ongoing and officers are appealing for witnesses to come forward and contact them at Newton Stewart on the 101 number if they saw anything which might help.