The 19-year-old became the first Monday qualifier to win on the LPGA Tour since 2015.
“I’m really proud of myself because I really worked hard for the trophy,” Wannasaen said.
“I don’t give up. I always work hard to improve my short game, improve my mind, improve my play. I do everything to get a trophy.”
Sarah Kemp was the best Australian, carding four birdies and a bogey during her final-round 69 to leave her tied for 21st at 13-under.
Grace Kim (70) was a shot further adrift after an eventful first seven holes on Sunday, in which she made two bogeys, two birdies and an eagle before finishing two-under for the day.
Wannasaen arrived in Portland ranked 367th in the world and having missed nine straight cuts in her rookie season.
But she put together four stellar rounds in the 60s at Columbia Edgewater and shattered the tournament scoring record by five strokes with a 26-under-par 262 for her maiden LPGA Tour victory.
Wannasaen is just the third qualifier to win on Tour, after Canada’s Brooke Henderson (Portland Classic, 2015) and American Laurel Kean (State Farm Classic in 2000).
The Thai youngster was never really threatened in the final round and comfortably beat Xiyu Lin of China, who closed with a 64 for a 22-under 266.
China’s Ruoning Yin (64), American Gina Kim (66) and Spain’s Carlota Ciganda (67) were joint third another stroke back.
Tied for second place entering the final round, Wannasaen charged into the lead with birdies at the 3rd and 4th holes, an eagle at the par-5 5th, and two more birdies at the 6th and 7th – a stretch of six-under that catapulted her into the lead.
“I don’t give up. I always work hard to improve my short game, improve my mind, improve my play. I do everything to get a trophy.” – Chanettee Wannasaen.
She carded a 30 on the front nine and was never really pressured on the inward half.
She increased her advantage with consecutive birdies at 13 and 14, and added another at the 17th to increase her lead to five stokes.
One of few hiccups in her final round came at the 72nd hole when Wannasaen, who hit 13 of 14 fairways, powered her approach long and over the green – one of three missed greens in regulation in the final round.
She chipped short, but punctuated her bogey-free round and victory with a 10-foot par putt.
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