In an enthralling final day at TPC Sawgrass in Ponte Vedra Beach, Thomas produced a decisive burst around the turn to set up a one-shot victory over Westwood in the PGA Tour’s $15 million flagship event.
Helped by a brilliant run of an eagle and three birdies in four holes, the 27-year-old signed off with a 68 to add to a 64 on Saturday as he landed a $2.7 million top prize with a 14-under-par 272 total.
Westwood, who had started the day with a two-shot lead, birdied the last for a 72 to finish second, one ahead of both DeChambeau (71) and left-hander Brian Harman (69).
“I fought so hard today,” said Thomas as he savoured his 16th PGA Tour title triumph and biggest since landing the US PGA Championship in 2017.
“Probably one of the best rounds of my life tee to green, total control of the ball, and really was hitting a lot of good putts, just wasn't holing anything and I stayed really patient.”
Thomas, who was axed by clothing company Ralph Lauren over a homophobic slur during an event in Hawaii in January, was overcome with emotion when he was asked about his grandfather Paul, who passed away last month. “I wish I could talk to him. I don't know, it's a sign that he was watching,” he said.
Albeit from a hospital bed in Los Angeles following the serious leg injuries sustained in a car crash, Tiger Woods would certainly have been watching his close friend come out on top in the game’s so-called fifth major.
“I was replaying what he told me a lot in my head,” said Thomas of the 15-time major winner.
“I always appreciate his help. We're all pulling for him, and I'm so glad to hear everything has been going well with him. But part of me wishes he was here so I could rub it in his face a little bit more.
“He's so nice to myself and Bryson and so many guys out here that if you would have told us when we were 15-20 years old that Tiger Woods was texting us the night before we have a chance to win the tournament trying to inspire us, that's pretty cool.”
Westwood holed a 12-foot birdie putt on the closing hole to finish second on his own for the second week after being pipped by DeChambeau in the Arnold Palmer Invitational at Bay Hill the previous weekend.
“The week has been great,” said the 47-year-old Englishman. “I didn't play my best golf today, but I battled it out. I was proud of myself for that.
“People question whether I can hole putts under pressure, and I rolled in a lot of must-make putts all day.
“I'm just having so much fun. Everybody keeps telling me how old I am. I'm 48 in a month's time, and I'm still out here contending for tournaments and playing in final groups with great players like Bryson DeChambeau and Justin Thomas
“It's just a joy to be involved and still playing well and being able to contend.”