Dan Clement, 102, performed the honour for his friend Sandy Little when he married Susan Davey.
The couple travelled from Cornwall for the ceremony near Mr Clement's home in Bridge of Dee, near Castle Douglas.
The current record-holder is Ronald Hornby, who was 96 when he served as best man in 2014.
Mr Little, 73, said he had known Mr Clement for most of his life, and he was first choice for the vital role.
"I didn't even consider his age, I would have asked him anyway," he said.
"Whatever age he'd been, if he'd been 120 I would have asked him.
"He's always been an awful good support to me and an inspiration, I'd say."
Mr Clement, a retired farmer, said he did not have to think for long when he was asked to take up the role.
"I said I'd be delighted to be his best man," he said.
The wedding was held at the Mission Hall in Bridge of Dee on Wednesday, just 20 yards from Mr Clement's home.
During the ceremony, he successfully handed over the bride and groom's rings and signed the register as a witness.
Afterwards, he said: "I thoroughly enjoyed it."
Asked if he made any mistakes, he said: "No mistakes. None that I know of."
A spokesman for Guinness World Records said the current record had stood since 2 January 2014, when Mr Hornby was best man at the wedding of Isobel Coote and James Beattie in Clogher, Northern Ireland.
He added: "We would need to receive a record application from Dan Clement or his family so we can look into his claim and validate this as a record."
A Scottish centenarian is expected to enter the record books after laying claim to becoming the world's oldest best man.