Wesley Bryan captured his first PGA Tour title in the RBC Heritage at Harbour Town Golf Links.
England's Luke Donald finished second for the fifth time in nine years
He started the final round four shots behind leader Jason Dufner.
The former world number one, now 96th in the rankings after five years without a victory on a major tour, looked to have blown any chance of winning when he drove out of bounds on the second and ran up a double bogey.
However, he proceeded to card five birdies - including a chip-in from the bunker at the 11th - to move into a share of the lead.
Bryan holed a birdie putt from inside five feet at the 15th to go ahead once more, and Donald could not find a gain down the closing stretch to force a play-off; another errant drive on the par-five 15th forced him to chip out sideways and his 35-footer at the last missed.
American Bryan had six birdies and two bogeys in a closing 67 to finish on 13 under, with Donald a shot further back after a round of 68.
"I'm going to remember this for a long time," said Bryan. "I feel like I was missing greens and scrambling but still managed to make a good score of it.
"Looking at the leaderboard and knowing I just needed to make a two-putt on the last was pretty special."
Graeme McDowell finished in a tie for 29th after shooting a 70 while Shane Lowry also failed to make any significant progress with a one-over round of 72 to end up joint 44th.
Patrick Cantlay, William McGirt and Ollie Schneiderjans were tied for third on 11 under, but former US PGA champion Dufner had a day to forget as he plummeted down the leaderboard with a closing 76.