Koepka made six birdies Sunday — including three straight on the back nine — and never let the pressure of the big stage deter him en route to winning the 117th U.S. Open at Erin Hills.
Koepka's 16-under total tied Rory McIlroy's record (2011, Congressional) for lowest score in relation to par at a U.S. Open.
Koepka started strong with birdies at 1 and 2, then added another at the 8th, all along leaning on words of encouragement he had received in a phone call from his close friend Dustin Johnson on Saturday evening.
Harman, who has won twice on the PGA Tour, most recently at the Wells Fargo Championship last month, played the opening nine in one under but fell back with bogeys on 14 and 15.
Rickie Fowler started the day two shots off the lead and looking to win his first major title, but he couldn't catch Koepka.