Reigning US Amateur champion Nick Dunlap became the first amateur since 1991 to win a PGA Tour title, winning a back-nine showdown on Sunday to capture the American Express tournament.

Dunlap, a 20-year-old sophomore at the University of Alabama, sank the winning par putt from just inside six feet at the 18th hole to fire a two-under-par 70 at the Pete Dye Stadium Course – one of three courses used this week.

Despite his one-stroke victory, however, Dunlap will not take home any money from the win.

As an amateur, the American is ineligible for the winner's prize of US$1.51 million, which goes to runner-up Christiaan Bezuidenhout of South Africa.

Dunlap finished on 29-under 259 to set a 72-hole record low winning score, breaking the mark of 28-under set by Patrick Reed in 2014.

"I felt this script today was already written," Dunlap said. "I was going to give it everything I had whether I shoot 75 or 65 or 70."

Dunlap became the first amateur to win a US PGA Tour event since Phil Mickelson captured the 1991 Northern Telecom Open in Tucson, Arizona.

Dunlap also became the second-youngest PGA Tour winner in the past 90 years, trailing only Jordan Spieth's victory at 19 at the 2013 John Deere Classic, and the youngest amateur winner since 1910. (AFP)