BANGKOK, 14 Jun 2015 (AFP) - The two Koreans were tied for much of the day until the last two holes. But Kim, better known as "KT", remained calm under pressure to sink two birdies and finish with a seven-under-par 65.
Wang, 19, saw his hopes for silverware dashed after he shot a par and a bogey on the last two holes.
Those costly errors allowed Kim to cruise to victory in the $1 million tournament with a total of 267 compared to Wang's 270.
Following them was Australia's Joshua Younger, who began the final day at the start of the pack following a remarkable third day where he shot to the top with 64.
It was a feat he was unable to replicate on day four, finishing with 73, the weakest of his four rounds.
Japan's Yusaku Miyazato shared fourth place with local favourite Jazz Janewattananond, the highest placed of the Thai players, with 275.
Fellow countryman Chapchai Nirat took the consistency crown, shooting 69 on all four days to take joint fifth.
Kim's win ends a three-year wait for a title. The last time the 28-year-old lifted silverware was in 2012 at the Fujisankei Classic.
"Today I had luck with me that made me a champion," he said after the match, according to a report by the tournament organisers.
Kim put his lack of wins in the last few years down to changing his swing while trying to make it in the United States.
"This year I asked my coach to help me and he reminded me of the swing I had last time. Now I have a good mental image of my swing -- so that is why I am doing well," he said.
Wang said he felt little disappointment losing out to his older compatriot on the closing straight of a two-way race.
"I played really good today. I do not have any regrets. I had a little pressure the whole day but not that much," he said.
Final scores from the co-sanctioned OneAsia/Japan Golf Tour Organisation Thailand Open:
267 - Kim Kyung-Tae (KOR) 71-64-67-65
270 - Wang Jeung-Hun (KOR) 69-70-64-67
274 - Joshua Younger (AUS) 67-70-64-73
275 - Yusaku Miyazato (JPN) 72-68-67-68, Jazz Janewattananond (THA) 69-66-68-72
276 - Hiroshi Iwata (JPN) 74-62-72-68, Kim Dae-hyun (KOR) 71-69-68-68, Chapchai Nirat (THA) 69-69-69-69, Prom Meesawat (THA) 69-68-67-72