But Koepka appeared to be on course for the biggest choke ever recorded at the majors when his lead was cut significantly by a barnstorming Johnson.
As Koepka bogeyed the par-three 14th and Johnson had birdied the 15th, Johnson came within one shot of his fellow long-hitter.
Despite finding a horrible lie on the lip of a fairway bunker on the 18th, Koepka salavaged a miraculous par and has now won four of the past six majors played on US soil.
No player has achieved that feat since Tiger Woods between his historic run between 1999 and 2001.
‘‘I’m just glad we didn’t have to play any more holes, That was a stressful round of golf,’’ Koepka said on the 18th green.
‘‘I’m still in shock. I don’t know if I dreamed of this but this is so cool.’’
He is the first repeat winner of the PGA Championship since Tiger Woods accomplished the feat in 2007.
Six-time major winner Nick Faldo was astonished at Koepka’s dominance, becoming the first male in golfing history to win his first four major titles in less than two years.
‘‘Winning four majors out of eight he’s played … 50 per cent? That for me is dominance. Some seriously unbelievable dominance,’’ Faldo said.
Meanwhile, Adam Scott’s 74 sank him to one over but he still earned a share of eighth.
Fellow Australian Jason Day, the 2015 PGA champion, closed with a 72 to drop to four over and tied for 23rd.
Cameron Smith’s 74 gave him an 11-over total and Lucas Herbert (75) finished a stroke further behind.
BROOKS KOEPKA’S PGA CHAMPIONSHIP TRIUMPH BY THE NUMBERS
* Secures his fourth major victory, to go with the 2017 and 2018 US Open titles as well as the 2018 PGA Championship
* Becomes the first player in history to win the US Open and US PGA Championship back-to-back in a career
* Joins Tiger Woods (1999-2000 and 2006-2007) as the only players to win back-to-back PGA Championships since it moved to stroke play in 1958
* Becomes the first male golfer in history to hold two different majors back-to-back at the same time, and the first to win his first four majors in less than two years
* Becomes the first player since Hal Sutton in 1983 to win the PGA Championship in wire-to-wire fashion
* Becomes the only player under the age of 30 to have four major wins
* Joins Rory McIlroy as the only player under age 40 with at least four majors
* Moves to world No.1 again, his fourth stint at the top of the rankings
* Wins $US1.98 million winner’s share of the $US11 million PGA purse
* Koepka’s four-over score was the highest final round in relation to par by a winner of a major since Vijay Singh shot 76 to win the 2004 PGA Championship (Singh won in a play-off).