A trio of Koreans were grouped in a tie for third, with defending champion Park Sangh-yun (73), Park Hyo-won (72) and Kim Seung-hyuk (66) all sitting four strokes adrift of Lee at Namseoul Country Club.
The 18-year-old Khongwatmai, bidding to become the OneAsia Tour's youngest winner, ended up tied in sixth after a error-strewn round that saw him rack up seven bogeys and a double-bogey on the final hole in a five-over 76.
Lee started unevenly, hitting two birdies and three bogeys on the front nine but dug himself out of trouble with eagles on the par-five fifth and ninth holes to reach the turn with a one-shot lead over Khongwatmai and Moon.
Moon, who started five shots behind Khongwatmai, reeled off four birdies to draw level with the Thai teenager at the turn, but had to settle for second after recording two bogeys against a lone birdie on the back nine.
Lee settled into a rhythm after his turn, making five par saves in a row before a birdie on the 15th helped him open up a two-shot lead over Moon.
Lee dropped a shot on the penultimate hole but Moon's error on the 16th ensured he still had a two-shot cushion going into the final hole and he held his nerve for a par to seal the win.
(Reporting by Simon Jennings in Bengaluru)
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