PRAGUE, 22 Aug 2016, (AFP) - Belgian Pieters led by two strokes overnight from Peterson, who had catapulted himself into contention with a 64 on Saturday.
The world number 398 carried that form into the final day in Prague, as he rolled in seven birdies in a five-under-par 67 to take his maiden professional win on 15 under par overall.
"This is the best feeling in the world," he told the European Tour website. "My team and I have put in so much hard work and long hours by everybody and I'm just so happy.
"We had this as a goal this year and to be able to accomplish it means so much.
"It was fantastic, the crowds have been great all week. My sports psychologist was on the bag this week and my coach and my girlfriend came all the way from Phoenix. It's just so great to have all the team here."
Peterson drew level with the 24-year-old Pieters after the par-five opening hole at the Albatross Golf Resort, as he birdied while his playing partner took a six.
The seesaw nature of the final round continued, and although Pieters was the beneficiary of a two-shot swing on the second green, Peterson found three more birdies to take a one-shot lead at the turn.
Peterson, whose previous best finish on the European Tour was a tie for fifth in Malaysia last year, slipped back with a bogey on the 11th, but a birdie at 16 was enough to give him a single-shot advantage going to the 72nd hole.
Pieters, who had to win in the Czech Republic and next week in Denmark to make Darren Clarke's Europe Ryder Cup team automatically, appeared to have forced a play-off when he hit his approach shot to within three feet on the par-four last.
But Peterson, 28, poured in his birdie effort from 17 feet to deny the world number 61 a third European Tour title.
Two-time Ryder Cup winner David Howell finished four shots behind the champion in a share of third place with fellow Englishman Ryan Evans.
Matthew Fitzpatrick, who went into the week in the final automatic European spot for the Ryder Cup matches that start next month at Hazeltine, all but secured his place by finishing fifth on ten under.
Leading scores from the final round of the European Tour's Czech Masters at the par-72 Albatross Golf Resort in Prague on Sunday (GBR & IRL unless stated):
273 - Paul Peterson (USA) 72-70-64-67
274 - Thomas Pieters (BEL) 67-67-70-70
277 - David Howell 70-68-71-68, Ryan Evans 66-73-70-68
278 - Matthew Fitzpatrick 69-69-71-69
279 - David Lipsky (USA) 71-70-68-70
280 - Gary Boyd 72-73-66-69, Robert Karlsson (SWE) 68-70-71-71, Graeme Storm 68-72-68-72, Robert Rock 72-70-65-73
281 - Justin Walters (RSA) 72-71-69-69, Joachim B. Hansen (DEN) 77-68-67-69, Steven Brown 69-69-73-70, Marc Warren 71-68-72-70, Mark Foster 72-70-69-70