The victory was the 29-year-old’s second major after his first win at the 2017 PGA Championship at Quail Hollow.
Both players entered the playoff tied at -5 but the results could have been very different if Mito Pereria did not suffer his misfortune.
Pereira was the 54-hole leader when he entered the 18th hole with a one-stroke lead over Zalatoris and Thomas.
But after his tee shot found the water, Pereira carded a double-bogey to miss the playoff entirely by one.
The 27-year-old became only the third player in the past 20 years to double-bogey the last hole of the final round in a major to miss the playoff by one with Phil Mickelson and Colin Montgomerie both doing so at the 2006 U.S. Open.
Thomas’s comeback was remarkable given he was seven shots back of the lead entering Sunday’s final round before firing a 3-under 67.
He tied the largest final-round comeback at the PGA Championship on Sunday with seven after John Mahaffey did so in 1978.
Thomas opened the tournament with back-to-back rounds of 67 to make it to the weekend, but a third-round 74 on Saturday knocked him back down the leaderboard.
After making two bogeys and two birdies on the front nine Sunday, Thomas went bogey-free on his back nine, making three birdies en route to the 67.
Both Zalatoris and Thomas birdied the first playoff hole, the par-5 13th before the latter was able to grab the edge over the 25-year-old with a second birdie at the par-4 17th.
Justin Thomas then secured the PGA Championship with a par on the final playoff hole.
Thomas now has 15 wins on TOUR but before Sunday’s win, it had been over a year since the Kentucky-born player had a win on TOUR with his last win coming at The Players Championship in March 2021.