Kiradech, who tied with Welshman Jamie Donaldson and England’s Laurie Canter, had been the first to set the clubhouse target after covering his first 16 holes in nine under par, only to bogey the 17th after almost hitting his second shot out of bounds.
Billy Horschel won the tournament, becoming only the second American after Arnold Palmer to lift the trophy.
Horschel hit seven birdies to finish 19 under par, putting him one shot ahead of Canter, Donaldson and Kiradech on the final leaderboard.
The 34-year-old follows the legendary Palmer as an American winner of the European Tour’s flagship event, with his compatriot claiming the silverware at Royal St George’s in 1975.
Donaldson birdied the 17th and 18th to complete a 66 and join Kiradech in top spot before Horschel birdied the last to edge in front.
Canter was unable to find a birdie on the 17th or 18th to force a play-off, the 31-year-old thinning a chip from just short of the green on the 17th and narrowly missing from 15 feet on the last.
“Man, I’m speechless, which is very, very rare. The crowds are absolutely unbelievable here and they supported me when I came over in 2019 and they supported me this year. To win this event means a lot,” Horschel told Sky Sports.
“In my mind, it’s equivalent to The Players Championship. We have a great event on the PGA Tour and I live there and would love to lift that one, but I have this one under my belt and I’m very happy.”
Horschel won the WGC-Dell Technologies Match Play in March but was overlooked for a United States Ryder Cup wild card last Wednesday.
He did not even receive a call from US captain Steve Stricker, a snub that still lingered and provided motivation for Horschel at the PGA Championship.
“It sucks not making the team. I didn’t play consistent enough and well enough after I won the Match Play to warrant a pick or to get enough points to be an automatic selection,” Horschel said.
“I was a little gutted I didn’t get a call this week. I didn’t think the call was going to say I made the team, but I was a little gutted that I didn’t get a call to say, you didn’t make the team.
“In my mind I thought I would at least get that. There was a little more added motivation this week for that.”