ERIN HILLS, 18 JUNE 2017, (European Tour) - The American entered the final day one shot behind overnight leader Brian Harman and the two were engaged in a real battle for the title before Koepka made three birdies in a row from the 14th to move clear at 16 under.
Harman signed for a 72 to emerge as the closest challenger alongside Japan's Hideki Matsuyama, one shot ahead of England's Tommy Fleetwood, who also carded a closing 72.
The victory is Koepka's second on the European Tour after the Turkish Airlines Open in 2014, a season in which he finished eighth in the Race to Dubai Rankings presented by Rolex.
His 16 under par winning total also matches the US Open record of Rory McIlroy when he won his first Major at Congressional in 2011.
"I was way more confident this week than I've ever been, I felt like I was striking the ball really well, I was putting the ball in the fairway - which you need to do out here - and then I was putting so well." he said.
"It's for all the hours that you put in and things like that. You look back a couple of years ago to be on the Challenge Tour, four guys driving around in a little mini car, four golf bags packed in there, to be the US Open champ is pretty cool.
"To go over to Europe and kind of cut your teeth over there and be able to play in different conditions is what you need to do.
"I built on those wins on the Challenge Tour, any time you can win you're going to have confidence and I look back at those."
Americans Rickie Fowler, Bill Haas and Xander Schauffele were at ten under, a shot clear of Charley Hoffman and two ahead of Trey Mullinax, Brandt Snedeker and Justin Thomas.