The American world No.14 Thomas, previously regarded as one of the best players without a major, broke through to claim the final major of 2017 by two shots and his fourth US PGA Tour win of the season.
Matsuyama, who went into the final round one shot off the pace, finished with a 5-under 279 total at Quail Hollow, with a wild three-birdie, five-bogey final nine ending his charge.
"It just kind of snuck up on the hole", Thomas said.
This year's edition of the US PGA Championship has been a war of attrition for players and viewers alike. "To do that at the PGA Championship is pretty special", DeLaet said.
The pair lie on five-under par for the tournament.
"Look, I don't know what I'm going to do", he said. "But the end of the day, this was really cool".
Kevin Kisner was the last one who had a chance to catch him.
"Yeah, the gravity took over and the roar was pretty loud, so that was pretty cool".
Matsuyama, looking to become the first Japanese player to win a major, got through the front nine in a one-under 34 to be seven under overall.
Thomas' round gained even greater momentum when on the 10th, he rolled a birdie putt towards the hole where it clung to the lip for 15 seconds before eventually dropping. The birdie gave him a two-shot lead.
Eventual victor Justin Thomas stroked an 8-foot putt for a badly needed birdie on No. 10 and appeared to leave it hanging on the lip of the cup.
He recovered to hit onto the green with his third, and closed out his round with a bogey to finish on 8-under. Three birdies coming in made things interesting, but they were paired with three bogeys and unlike Thomas it never felt like Kisner was going to be able to pull away from the crowded back at the top.
Day was four shots behind Kevin Kisner playing the last hole Saturday when he blocked his tee shot into the pine straw. "And that chip-in on 13 was huge", Thomas said.
Italy's Francesco Molinari (67), South Africa's Louis Oosthuizen (70) and American Patrick Reed (67) finished at 278. All finished at 278.
That was followed by a chip-in from the fringe on No. 13 for another birdie en route to winning the Wanamaker Trophy.
Rickie Fowler (67) and Japan's Hideki Matsuyama (72), who was aiming to win for the second week in a row, were next at 279.
A full twelve seconds after the ball came to rest, gravity took over and the ball dropped, giving Thomas his third birdie in four holes as he made his move up the leaderboard.
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