Taking a four-stroke lead heading into the final round, the 17-year-old hit a five-under-par 67 to finish on 201 at Kabin Buri Sports Club in the eighth tournament of the 2020 Thai LPGA Tour season.
Kultida Pramphun, 19, also shot a 67 to finish second on 205 in the B1 million event.
Amateur Tunrada Piddon was third on 208.
Atthaya, who picked up five birdies in a bogey-free round, sealed her third straight win and fourth overall on the Thai LPGA Tour in eight appearances.
“I am happy with the win. I am satisfied with my improvement since turning pro in February,” said the Ratchaburi native who won twice on the Ladies European Tour when she was amateur.
“But there is still room for improvement.”
Atthaya took home B120,000 in prize money and was crowned the 2020 Order of Merit winner with a total earnings of B697,650.
Kultida received B69,000.
The next tournament, of which prize money is not counted in the Order of Merit, is the B4mn Thailand LPGA Masters which will be held on Nov 18-20 at Watermill Golf and Resort in Nakhon Nayok.