The 17-year-old shot a five-under-par 67 in the final round at Kabin Buri Sport Club for 199 in the 54-hole tournament.
In her fourth tournament as a pro, the Ratchaburi native powered past overnight leader Dolnapa Phudthipinij, who carded a 70 for 201.
Atthaya took home B150,000 in prize money from the total purse of B1 million while Dolnapa got B89,000. The young star won two Ladies European Tour events in 2017 and 2019 when she was an amateur.
“I am really happy to win in my only fourth tournament as a pro,” said the former world No.1 amateur.
“I came here to improve my game. Winning the title proved I am on the right track and my hard work has paid off handsomely.
“I hope this is the beginning of a bright career.”
The teenager said she had learned from her failure to deliver at the previous Thai LPGA Tour event in Hua Hin two weeks ago when the domestic season restarted after the COVID-19 break.
LPGA Tour player Pavarisa Yoktuan, who hit a hole-in-one at the 154-yard eighth hole, shared third place with Busabakorn Sukhapan on 203. They each got B59,400.
The fourth leg of the Singha-SAT Thai LPGA Championship season will be held at Watermill Golf Club and Resort in Nakhon Nayok from Aug 6-8.