Keith Mitchell holed a testing 15-foot birdie putt on the final green to snatch a one-shot victory at the Honda Classic in Florida.
The 27-year-old American held his nerve in the closing stages at PGA National to edge out Brooks Koepka and Rickie Fowler, carding a three-under 67 to secure his maiden PGA Tour win.
Mitchell, the world No 162, had appeared out of contention when he bogeyed the opening two holes in the final round, but he regained his composure to emerge from a logjam at the top of the leaderboard thanks to birdies at the 12th, 13th, 15th and 18th, which took him to nine-under 271.
Ryan Palmer looked at one stage as though he would make a record-breaking comeback after coming from nowhere to take the early clubhouse lead with a seven-under 63 - the best round of the week - to finish on seven under.
However, he was eventually overtaken by Mitchell, as well as Koepka (66) and Fowler (67), who also birdied the 18th.
Three-time major winner Koepka has flown a little under the radar this week but three birdies on the front nine put him firmly in the mix and, after a bogey at the 11th, he finished with birdies at the 16th and 18th to become the first man in the clubhouse on eight under.
After birdies at the third and fourth, Fowler slipped up with bogeys at the sixth and seventh, the latter after a horrendous tee shot, but he proved his mettle with birdies at the 15th, 17th and 18th to join Koepka and Mitchell on eight under.
Mitchell still had the last to play, though, and despite finding a bunker off the tee he was able to find the centre of the green with his third shot before holing the crucial putt, after which he told Sky Sports: "It's not even close to sinking in. I don't expect it to, I don't want it to, this is just awesome. Playing like I did coming down the stretch is unbelievable."
Palmer admitted after completing his round - he teed off three-and-a-half hours ahead of the overnight leaders - that he would have liked to get to eight or nine under and he missed a relatively easy birdie chance at the 18th following gains at both 16 and 17.
He had earlier completed the front nine in just 29 blows after embarking on his round with four successive birdies and adding another two at the eighth and ninth.
After starting in a tie for 39th place, Palmer would have completed the greatest comeback in terms of position in PGA Tour history had he maintained his place at the top of the leaderboard after holding the outright lead at one point following overnight leader Wyndham Clark's stumble.
Clark began the day with a one-shot cushion and remained calm over the first six holes as two birdies took him two clear on nine under.
However, a three-putt bogey at the seventh saw him hit the buffers before further dropped shots at the eighth, 11th and 12th brought him back into the pack.
The rookie eventually signed for a 72 after bogeying the 18th, leaving him in a tie for seventh place on five under with South Korea's Kyoung-Hoon Lee (71).
Lucas Glover had earlier joined Palmer on seven under after he produced a birdie-birdie finish for a 66, with the pair finishing one ahead of Vijay Singh.
Singh, 56, failed in his bid to become the oldest winner of a PGA Tour event as he mixed four birdies with four bogeys in his level-par 70, some tentative putting letting him down.