Fleetwood, who was hoping to become the third English victor since 1924 after Tony Jacklin (1970) and Justin Rose (2013), began the day in a tie for second and made an encouraging start with a birdie from eight feet on the second.
A three-putt bogey from Harman on the 13th left clubhouse leader Matsuyama as Koepka's nearest challenger, but the American promptly got up and down from a bunker on the 14th and holed from 12 feet for another birdie on the next.
"Two weeks ago, just before I came up here, I spoke to one of the course designers, Ron Whitten", Nobilo said on Golf Channel's Live From the U.S. Open show Sunday night, "and I said, 'can you give me an estimation on what might the winning score be here?' He said, 'if the wind blows, four under".
Brian Harman, who was the 54-hole leader by a stroke, didn't lose it.
On the last day of the U.S. Open, Hideki Matsuyama was better than everyone else. The only drama at the end was whether he'd make birdie on the par-5 18th to break Rory McIlroy's scoring record in relation to par.
Koepka's three-putt bogey on the 10th gave the chasing pack renewed hope, but Fleetwood was unable to convert birdie chances from six feet on the 11th or twice the distance on the next.
With a significant lead going into the final two holes, the two easiest on the course, Koepka admitted to watching the leaderboard, but the sight didn't slow him down.
Cameron Champ was the lowest amateur heading into the final round, but he finished with a 4-over 76 to drop back to even par for the tournament. It was an accomplishment that wasn't lost on golf's newest major champion, even if his father and other family members weren't in attendance at Erin Hills on Sunday and missed one of Koepka's appearances at a major for the first time.
The 27-year-old American, who had to leave his home country and join Europe's minor-league tour to get his career going, won the U.S. Open.
Heading into the 12th hole, Harman and Koepka were tied at 13-under.More news: Did Lebron James deserve to be voted NBA Finals MVP?
However, Harman eventually stumbled with back-to-back bogeys on 12 and 13 and his rival found peak form at the ideal moment - birdieing three holes in a row from the 14th to all but wrap up the tournament.
The streak continues. For seven straight major we have a first time major champion.
He started his final round one shot off the lead.
England's Paul Casey featured in a four-way tie at the top of the 117th US Open leaderboard on Friday as the big guns crashed out of contention.
Two Wisconsin natives competed in the U.S. Open. "He's just like Dustin, I would say". "I played really solid from the moment I got here".
"I feel like I'm playing at the highest level", Fowler said.
"I was pretty content making pars on the front nine because I knew the kind of day it was", Harman said.
This week alone, nine players reached at least 10 under and seven finished there. Three wins on the second-tier Challenge Tour over two months in 2013, earned him the right to jump to the European Tour. Harman did not fade or choke at all in what presented a totally new challenge in 20 miles per hour winds compared to the first three favorable scoring days at Erin Hills.
Brooks Koepka may confirm the biases that many had coming into this U.S. Open that Erin Hills would set up as a bombers' paradise. It was obviously playing a lot more hard, so I gave it all I had. "I think it's an awesome golf course".