The 27-year-old American, who cut his pro teeth on Europe’s second-tier tour, triumphed by four shots over fellow American Brian Harman and Japan’s Hideki Matsuyama at Erin Hills, Wisconsin, Sunday.
The powerful Floridian fired a final-round 67 to become the seventh first-time major winner in a row and third consecutive home winner.
Koepka’s winning score to par of 16 under equals the mark set by Rory McIlroy in 2011, although Congressional Country Club was a par 71, not 72.
The world No. 22, a member of the victorious US Ryder Cup team in 2016, pocketed a cheque for $2.16 million alongside the US Open trophy.
“It’s unbelievable, it’s pretty cool,” said Koepka, who has knocked on the door before in majors, with fourth in the 2014 US Open and 2016 US PGA.
How did Koepka win the title?
Jay Townsend said in commentary that Koepka only seriously got into golf after being involved in a car accident at the age of 10.
Townsend added: “As a result, he was banned from playing contact sports and that’s how he seriously got into golf. It was kind of by accident.”
The 27-year-old from Florida who turned professional in 2012, had won twice on the European Tour and just once on the PGA Tour before this win, which earns him $2.1m (£1.6m).
Koepka opened his final round in perfect fashion with birdies on his opening two holes. Another followed on the eighth and he battled hard after dropping a shot on the 10th, holing a 10-foot par-putt on the 13th before effectively winning the title with his birdie streak from the 14th.
The statistics show that he won with a combination of power and accuracy from tee to green on the 7,845-yard course, the longest in major championship history.
“You have to measure success in different ways, not just by winning, just because that doesn’t happen a whole lot,” said Fowler.
“I think Tiger had the best winning percentage of all time at 30%, and you’re lucky to even sniff close to 10.”
Elsewhere, Masters winner Sergio Garcia couldn’t parlay his first major victory into a second and never made a Sunday charge despite shooting under par in each of the first three rounds. He stayed at four-under par for the tournament with an even 72.
That cleared the way for Koepka, who sprinted to the win with his late streak of birdies.