Koepka carded a closing 66 in a breathless final round at Bellerive Country Club to finish 16 under par and two shots ahead of Woods, joining the runner-up, Gene Sarazen, Ben Hogan and Jack Nicklaus as the only players to win the US Open and US PGA in the same season.
The 42-year-old American underwent spinal fusion last year but showed glimpses of the sublime form that has earned him 14 majors.
"I'm pretty disappointed with myself that I couldn't play the last three holes better".
Joining the 14-times major victor at eight under are American compatriots defending champion and world number two Justin Thomas, 2009 British Open victor Stewart Cink, Australian 2015 PGA champion Jason Day, Irishman Shane Lowry and South African 2011 Masters champion Charl Schwartzel, who matched the record low score in the 100-year history of the PGA Championship with a second-round 63.
Or go back to Thursday (Friday NZT), when a bogey and double bogey on his first two holes almost ended his tournament before it really began.
"I think it's really putting St. Louis on the map", said Decker.
Tiger Woods had to settle for second at the PGA Championship, despite his best-ever final round at a major.
"I was in contention in the last two major championships and I would never have foreseen that a year ago".
FBI fires agent Peter Strzok over anti-Trump texts
Strzok also faced off against Republicans during a 10-hour congressional hearing regarding the text messages in June. Former FBI lawyer Lisa Page arrives for a House committee deposition, one day after the session involving Strzok.
"I had to kind of figure this out on my own and it's been really hard - a lot harder than people think, " he said.
"To come out here and play as well as we did was really, I can't even put into words how well I played and I'm so excited for myself and my caddie and my team".
He joins Jordan Spieth, Woods, Nicklaus and Tom Watson as the only players since World War II with three majors before turning 30.
Scott hung around by making big putts, just like he hoped, and was tied for the lead until Koepka's birdies.
"He's won three majors now, so he's definitely winning the right ones", Scott said.
Crowds cheered the veteran through a thrilling round that saw him miss every front-nine fairway but take only 10 putts, scrambling from dirt, trees and bunkers to stay in the hunt.
In the search for two shots that could've made the difference, one could point to the bad drive on 17, the agonising miss on 11 or a 12-foot putt to save par that went halfway in but rimmed out on No 14 and briefly halted his momentum.
"It's tough to beat when the guy hits it 340 down the middle", Woods said.
"Going from that to even being able to play this year to this, it's absolutely mind blowing", Koepka said. He missed four months at the start of the year when a partially torn tendon in his left wrist, causing him to sit out the Masters. He outlasted good friend Dustin Johnson at Shinnecock Hills to become the first back-to-back U.S. Open champion in 29 years.