Show jumper Nick Skelton became Britain's second-oldest Olympic gold medallist in his seventh Games.
The 58-year-old, who initially retired 16 years ago after breaking his neck in two places, claimed individual gold after a six-way jump-off.
It is Britain's first individual show jumping medal since Anne Moore's silver in 1972, and adds to Skelton's win in the team event at London 2012.
The victory takes Britain's Rio tally to 23 golds and a total of 57 medals.
Joshua Millner, who claimed shooting gold in 1908 at the age of 61, is the only person older than Skelton to have won gold for GB.
"You always dream about it but when it actually happens it is real life," Skelton told BBC Radio 5 live.
"I have been thinking about this and planning it since London four years ago.
"I knew if I kept my cool this horse would do it. I knew if I did not make a mistake, he wouldn't either."
Skelton, riding Big Star, went clear in both opening rounds and did so again as the first to go in the jump-off.
His gold medal was confirmed when Canada's Eric Lamaze clipped the penultimate fence and had to settle for bronze.
Peder Fredricson of Sweden claimed silver, and Briton Ben Maher, riding Tic Tac, finished in 25th place.
Skelton's is Britain's second gold in the equestrian events following Charlotte Dujardin's victory in the individual dressage.
Tina Cook, Britain's three-time Olympic medallist, on BBC Radio 5 live: "He has waited a long time for this moment.
"He had one mistake on the very first day and has upped his game to the maximum. He has got better and better and the horse has shown some world-class jumping.
"If I'm being honest I thought we did have a strong chance of a medal - but I couldn't have said it was gold."
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