Tied for the lead with six holes to play, Koepka holed an 8-foot par putt on the 13th hole that gave him confidence with his stroke and momentum to pour in birdies on the next three holes to turn the final hour into a celebration of another young star in golf.
Koepka's US Open victory is now the seventh consecutive major that has been claimed by a first-time champion, and when taking into account the performances from the likes of Jason Day, Dustin Johnson and Rory McIlroy in Wisconsin, the streak could well be extended to eight at The Open next month. Not with Koepka - he of the one PGA Tour win, the 2015 Waste Management Phoenix Open - firing a tournament record-tying total of 16 under par at the 11-year-old course located 45 minutes northwest of Milwaukee.
Oklahoma State's Rickie Fowler tied for fifth place, finishing at 10 under par.
A changing of golf's old guard has been underway for some time but the 117th U.S. Open signalled that it can be considered complete with neither Phil Mickelson and/or Tiger Woods teeing up at major for the first time since the 1994 U.S. Masters.
Thomas, fresh off his historic Saturday 63, couldn't back up his third-round magic.
"You've got to put four good rounds together", Matsuyama said through an interpreter. He opened with a pair of tap-in birdies and putted for birdie on every hole but the par-3 13th.
"At the start of the week I was thinking five-under would maybe win this thing, and I was pretty far off", he said.
"That's probably one of the coolest things I've ever experienced and to do it on Father's Day, it's pretty neat", Koepka said.
Hideki Matsuyama, of Japan, shakes hands with his caddie after his fourth round of the U.S. Open golf tournament Sunday, June 18, 2017, at Erin Hills in Erin, Wis. "I had an opportunity today and I didn't get it done". Instead, he went birdie-birdie-birdie, pushing his lead to a more comfortable 4 strokes with two holes to play before coasting home with his foot on the brake to become the seventh straight first-time victor of a major championship.
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.
Harman kept pace until the eighth hole where Koepka made birdie to move one clear and never relinquished the lead after that. I felt like I was playing really good. He went from a tie for second at the beginning of the day to a tie for sixth. In the day's last hour, when even the smallest bobble under increasing pressure has thrown many a train off the tracks, Koepka was sensational. "I believe, based off of history, that I'm a top-tier player and I'll start playing like that shortly", he said. He was 100 feet away for birdie on No. 6 and burned the edge of the cup on that one, except that it ran by some 7 feet and he three-putted for bogey. And I'll win if I stay patient and just keep doing what I'm doing. "I come from a baseball family, and it's in my blood".