Resuming his weather-delayed round on the 12th hole Saturday, the Lehi resident made two more birdies and two bogeys to finish.
Tiger Woods shot a 4-under 66 Saturday to move himself into a tie for sixth at the PGA Championship.
But Koepka always stayed just out of reach of Tiger with playing partner Adam Scott putting most pressure on the American.
The double US Open champion and Scott were tied for the lead with four holes remaining, with Woods just one behind and two holes ahead, but Koepka then holed pivotal birdie putts on 15 and 16 and held on to claim his third win of the last seven majors by a two-shot margin.
Six birdies in his first 13 holes offset a solitary bogey and lifted Woods, who started the day 10th, to within a shot of the lead amid increasing excitement. It was his 100th start on the PGA Tour, and the win his third major title.
"I feel like, if I do what I'm supposed to, I should win the golf tournament", Koepka said.
Woods hit zero fairways on the front nine but was able to repeatedly pull off miraculous recovery shots, the same ones that made him so great in his glory days. He tied Gary Player's record for the most birdies in a round in the PGA Championship.
Despite the long, hot day, Tiger was feeling the love from the St. Louis crowd.
However, that didn't stop Woods from continuing to hit jaw-dropping shots to send the crowd into a frenzy.
McIlroy bogeyed the opening hole after flying his approach over the green and although he responded immediately with a birdie on the next, further dropped shots on the fourth and seventh saw him tumble down the leaderboard.
She accompanied Koepka as he walked off the 18th hole as the event champion. He outlasted good friend Dustin Johnson at Shinnecock Hills to become the first back-to-back US Open champion in 29 years. He's won the PGA four times, the last in 2007.
Woods' back nine was more mixed as he suffered bogeys on 10 and 12, but the 14-time major victor countered that with two further birdies after hitting four on the front nine for a 66, which took him to four under overall.
"I thought I played really clean cards, really clean rounds", said Woods, who made eight birdies and three bogeys over the course of the two rounds.
And, in many ways, it felt like old times for the 42-year-old - he of the multiple back surgeries who could barely swing a club a few years back, but has now contended on the back nine in consecutive majors, only to come up short, more agonizingly so this time than at Carnoustie three weeks ago.
"I can think of plenty of people along the way telling me I'll be nothing", he had said the other day, "working at McDonald's, doing things like that".