Dunlap won as an amateur at the American Express tournament in California in January and on Sunday took his first professional victory.
The American is the first golfer to win twice in a rookie PGA Tour season since newly crowned British Open champion Xander Schauffele in 2017.
The tournament uses an unusual scoring system that awards two points for birdies and five for eagles, but deducts points for bogeys or worse.
Dunlap, nine points adrift when the day began, made six birdies in his first 12 holes and then clinched victory with an eagle at 15 and a final birdie at 17 in a bogey-free round of 19 points for a winning 49 total.
"Never even thought that was a possibility," Dunlap said. "Really happy to be here."
It was enough to hold off compatriots Mac Meissner, Vince Whaley and Patrick Fishburn.
Whaley closed with a birdie to finish second on 47 points with Fishburn a career-best third on 46 and rookie Meissner fourth on 44.
Dunlap received a sponsor's exemption at The American Express and scorched to a third-round 60 to match the lowest round by an amateur in any PGA Tour event on his way to a one-stroke victory.
After becoming the first amateur to win a PGA title since Phil Mickelson at the 1991 Tucson Open, Dunlap turned professional the following week.
Dunlap sank a 56-foot eagle putt at the 15th to seize the lead on 47 points.
"After I made the putt at 15, we started playing with some serious adrenaline," Dunlap said.
At the par-4 17th, Dunlap dropped his approach to three feet for a birdie and a par on 18 was enough to seal the win.
