He used the pivotal stretch to pull away and finish the championships at 16-under 272.
Koepka carded a closing 67 at a windswept Erin Hills to finish 16 under par, matching the tournament scoring record set by Rory McIlroy at Congressional in 2011. "It hasn't sunk in, obviously, and (it) probably won't for a few days".
HARMAN: No, I don't think there's - I think most of the good players don't really compare themselves to their peers. "On some of these par fives I don't even need to hit driver to get there and I putted brilliantly this week too". He got off to the ideal start with a two-putt birdie on the par-5 first and a tee shot just short of the green on the 338-yard second hold, leading to a simple up-and-down for another birdie.
He followed with birdies at 15 and 16 to suddenly widen his gap to four shots and close in on the Open championship.
And the prime beneficiary was Thomas, who catapulted up the leaderboard with his record-breaking nine-under-par 63.
When Matsuyama, at number four the highest ranked player to make the cut, delivered the round of the day a six-under 66 to get within one of the lead, a rampaging Koepka countered with golf's version of the knockout punch, clinching victory in ruthless style by registering three straight birdies from the 14th. Harman bogeyed his second straight hole at 13 to fall two shots behind Koepka.
HARMAN: Yeah, well, even when I made the couple bogeys, I made the birdie right there at 14.
Kite says it looks as if the conditions at Erin Hills "are going to be a lot more challenging", but he doesn't think it will look like what happened 25 years ago. And sometimes when you're not seeing the ball go in the hole, it gets harder and it's easy to lose faith a little bit. Then Koepka went on his back-nine birdie barrage to put the tournament to bed.
"I'm proud of the way I hung in there today". At the same time, I don't feel as if I lost the golf tournament.
"I'm just going to keep trying to do what I'm good at and keep doing what I know how to do and we'll see where it takes me".
After his birdie at 14, Koepka wasn't aware of Harman's bogey back at 13.
"I was kind of, I don't want to say homesick, it was just, exhausted of golf".
Matsuyama has two holes left, compared to six for Koepka and seven for Harman.
Henderson won twice past year, taking the KPMG Women's PGA Championship for her first major, and successfully defending her title in the Cambia Portland Classic.
"After that, it was kind of lights out", Harman said.
Koepka, South Korea's Kim Si-woo, Japan's Hideki Matusyama, Englishman Tommy Fleetwood and Americans Brian Harman and Rickie Fowler were the names battling for major glory on Sunday, rather than the likes of Jim Furyk, Stewart Cink and Ernie Els.
His confidence in the final round came, in part, from a short conversation the night before with Dustin Johnson, a friend who had been through his share of close calls and heartbreaks before finally becoming a major champion at the U.S. Open a year ago.
"I felt like I did a lot of good things, especially in the first round, executing my game plan".
NOTES: Scottie Scheffler, who plays collegiately for the University of Texas, was the top amateur finisher at 287 for the tournament with a Sunday round of 73.
"Going over to play the Challenge Tour was really, really cool, to get to travel the world at 22 and do what you do for a living is pretty neat", he told reporters with the gleaming silver U.S. Open trophy by his side. He came into the U.S. Open with one Ryder Cup appearance under his belt, and three top-five finishes in majors. I asked Keegan Bradley's caddie when they brought the correct sheet out to their group (which included Jordan Spieth) on the 5th hole.