Thitikul — a stroke ahead after Hull bogeyed the par-4 17th — four-putted the par-5 18th from about 50 feet, missing a five-footer and a three-foot comebacker. Hull two-putted from about 30 feet for the victory.
"I wasn't really watching her putt for birdie because I thought she was going to hole it," Hull said.
"Then, yeah, I guess it's not over until the fat lady sings, but I was shaking over that last putt because I just didn't expect it."
Fighting ankle and back issues, Hull closed with a four-under 68 to finish at 20-under 268 at TPC River's Bend on Sunday. The Englishwoman won her third LPGA Tour title and first since 2022. She also has four victories on the Ladies European Tour.
Thitikul matched Hull with a 68. The Thai star — the winner in May at Liberty National in New Jersey, missed a chance to become the first multiple winner on the tour this season.
England's Lottie Woad (66) of England was third at 18 under.
Miyu Yamashita (69) was 17 under, and second-ranked Nelly Korda (70) followed at 15 under with Nasa Hataoka (65), Chisato Iwai (71), Sei Young Kim (69), Jennifer Kupcho (67), Mary Liu (70) and Maja Stark (69).
Robyn Choi finished as the leading Australian, in a tie for 12th at 14 under thanks to a final-round 65.
Minjee Lee (68) was a further shot back, with Gabriela Ruffels (73) 11 under, Steph Kyriacou (71) and Grace Kim (67) eight under and Hira Naveed (76) even.
Hull was coming off a runner-up finish in Houston last week on the European tour.
"It's been a rocky couple of months," the 29-year-old said.
"Obviously, fainting at Evian, doing my back, and then tearing the ligament in my ankle. I sound like a bit of a car crash. I don't know. I suppose pain is just a weakness of the mind. Probably does me a bit of a favour sometimes because I end up doing too much, too much gym, practice, overthinking. It's given me time to chill and that's probably what I needed."
With AAP.
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