Nick Dunlap held his nerve to become the first amateur to win a PGA Tour event since Phil Mickelson in 1991.
The 20-year-old holed a six-foot par putt on the last to claim The American Express title by one shot from South Africa’s Christiaan Bezuidenhout.
Because he is an amateur, the American will not receive any prize money, with the $1.5m (£1.2m) winner’s cheque going to Bezuidenhout.
“It’s so cool to be experiencing this as an amateur,” said Dunlap.
“Whether I made or missed that, if you had told me come Wednesday night that I had a putt to win this tournament, I wouldn’t have believed you.”
Mickelson, a six-time major winner, posted on X: “Such an impressive performance by Nick Dunlap. Congratulations on an incredible win.”
Dunlap – who had missed the cut in his previous three events on the PGA Tour – now has a two-year exemption on the circuit as well as entry to the Masters, the US PGA Championship and The Players Championship.
His stock was high even before this sensational victory after he emulated Tiger Woods last year by becoming the second player to win both the US Junior Amateur and US Amateur titles.
His final round at La Quinta’s Pete Dye-designed Stadium course featured a double-bogey six on the short par-four seventh after he pushed an iron shot off the tee into a lake as his three-shot overnight lead evaporated.
But the University of Alabama student responded magnificently with three birdies in his next nine holes as he closed with a two-under 70 to hold off a charging Bezuidenhout and become just the seventh amateur to win a PGA Tour event.
The South African – who holed a 138-yard wedge shot for an eagle two on the par-four 15th – also birdied the last in a seven-under 65 to set a clubhouse target of 28 under.
Dunlap, who had moved to 29 under with a birdie on the 16th, admitted he was unaware of Bezuidenhout’s final-hole birdie and, when he was standing over his second shot to the last, he thought he had a two-shot lead.
His approach leaked right and hit a spectator before fortuitously coming to rest a couple of yards off the edge of the green.
He chipped up to six feet and nervelessly holed the putt after his caddie told him “it’s inside left, you’ve made a million of these before”.
When asked if the next natural step was to turn professional, he replied: “I don’t know. I have to take a second to let what just happened sink in a little bit.
“That’s a decision that’s not just about me. It affects a lot of people, and obviously I’m going to try to enjoy this.”