World number 318 Jim Herman shot a closing seven-under-par 63 to win the Wyndham Championship by a stroke.
The 42-year-old American held off compatriot Billy Horschel to claim his third victory on the PGA Tour.
Kim Si-woo, the 2017 Players champion, was the overnight leader after a 62 on Saturday that included a hole-in-one but he finished three shots back.
England’s Paul Casey, second in last weekend’s US PGA Championship, finished tied 31st on nine under.
Ireland’s Shane Lowry, the Open champion, was 23rd on 12 under after a three-under-par 67, with America’s Webb Simpson, Doc Redman and Kevin Kisner joining Kim in third place.
“Obviously I’m ecstatic. I’m very emotional,” said Herman, who started the final day three behind Kim. “I came here this week with no expectations.”