Thomas won with a score of two-under after the three playoff holes with fellow American Will Zalatoris at Southern Hills Country Club at Tulsa, Oklahoma.
The 29-year-old finished his round a shot behind then-leader Mito Pereira, but after the Chilean hit his drive on the 18th hole into the water and finished with a double bogey, the major went to a playoff between the two compatriots.
And after three holes, Thomas ended with his second major victory as he lifted the famous Wanamaker Trophy for the second time having previously won it in 2017.
Thomas is one of the biggest names in golf.
Having burst onto the scene a few years ago though with 15 PGA Tour victories to his name, including his 2017 PGA Championship victory, he has not won an event since the Players Championship in March 2021.
But in recent months, there have been glimpses of him returning to somewhere near his best.
He finished tied for eighth at the Masters earlier this year and arrived in Oklahoma as one of the favorites for the title.
Back-to-back three-under par rounds Thursday and Friday cemented him near the top of the leaderboard, but a four-over par showing Saturday in testing conditions dented his title charge.
But he arrived on championship Sunday with a vengeance.
Once again, he shot a brilliant three-under 67 to put himself into contention at five-under for the tournament. When he finished his round, he remained a shot behind then-leader Mito Pereira.
However, disaster struck for Pereira on the last hole. Leading by a shot, the Chilean hit his ball in the water and eventually finished with a double bogey at four-under.
That catastrophic error forced a three-hole playoff between the two Americans. And under the low Oklahoma sun, the two did battle.
On the first playoff hole, the par-five 13th hole, it couldn’t have gone much better for both. After some beautiful iron shots and some solid putts, both carded birdies.
It was on the par-four 17th when Thomas’ first bit of brilliance gained him an edge.
A huge, booming drive through the bright blue sky had his ball on the green, leaving him two putts for a birdie. Zalatoris could only manage a par.
Thomas had a one-shot lead.
And on the par-four 18th hole, the final playoff hole, some calm and collected golf from the world No. 9 ensured a tap in putt from very close range for par, to wrap up the second major of his career.
With his dad Mike standing by and as the two shared an embrace on the 18th green, the emotions of the moment finally caught up to Thomas, now a two-time major winner.
He becomes just the sixth player since World War II with 15 PGA Tour wins including two majors before turning 30, along with Jack Nicklaus, Tiger Woods, Rory McIlroy, Tom Watson and Johnny Miller.
Thomas’ huge comeback to claim victory matches the largest comeback in the history of the PGA Championship of seven shots set by John Mahaffey in 1978.