It happened at around 4.10 pm just south of Dunscore, near Throughgate.
The man’s black Citroen Nemo van, which was travelling north, left the roadway and came to rest in a field.
Emergency services attended but he was pronounced dead at the scene. The victim has not yet been named by police.
The road was closed in both directions until about 9.45 pm for collision investigation at the scene.
Sergeant Jonnny Edgar of Police Scotland’s Road Policing Unit said: “Our enquiries are ongoing to understand why the van left the road and our thoughts are with the man’s family and friends at this time.
“The van was travelling north towards Dunscore and we’re eager to hear from anyone who may have seen it prior to the crash, or witnessed the collision itself.
“Any drivers recording on the B729 yesterday afternoon with dash cam are asked to check their systems and provide any relevant footage to us as soon as possible.”
Those with information should contact Police Scotland on 101 and quote incident number 2355 of 7 October 2021.