SEOUL, Oct 21, 2012 (AFP) - Kim, who had won the event twice before, scored a final-round two-under-par 69 for a five-under tournament total of 279 around the challenging Woo Jeong Hills Country Club in the central city of Cheonan.
He collected a winner's cheque of 300 million won (around $270,000).
Joint overnight leader Kim Dae-Hyun (71) birdied the last to grab sole second place.
Kang Kyung-Nam -- who led for two rounds before a 77 in the third -- fought back on Sunday with a 68 for a share of third with Major winner Y.E. Yang (67).
Kim Dae-Sub came to fame as a 16-year-old schoolboy amateur in 1998 when he beat the country's best professionals to lift his first Korea Open title.
He repeated the feat in 2001 and turned professional immediately afterwards. But he never quite fulfilled his potential and kept deferring his military service in an attempt to find form as a professional.
He was finally conscripted as a 29-year-old in 2010. Discharged just two months ago, he signalled his return by winning on the Korean Tour three weeks later.
"I always dreamed I would win the Korea Open as a professional but I didn't think it would happen this year," he said in comments released by the organisers. "This is the happiest day of my life."
The turning point in the final round came on the second hole when Kim Dae-Sub sank an 18-foot putt for birdie. He never looked back as playing partner Kim Dae-Hyun three-putted for bogey.
Japan's Ryo Ishikawa climbed up the leaderboard with four birdies on the front nine. But he was unable to sustain the challenge and had to settle for a share of seventh place, six shots off the pace, with American David Oh.
Rory Hie of Indonesia and Brazilian Lucas Lee -- who both finished at nine over -- were the only other non-Koreans in the top 30.
Leading scores after round four (par 71):
279 - Kim Dae-Sub (KOR) 72-68-70-69
281 - Kim Dae-Hyun (KOR) 71-73-66-71
282 - Y. E. Yang (KOR) 73-70-72-67, Kang Kyung-Nam (KOR) 68-69-77-68
283 - Ryu Hyun-Woo (KOR) 75-69-73-66
284 - Noh Seung-Yul (KOR) 73-74-68-69
285 - David Oh (USA) 73-72-72-68, Ryo Ishikawa (JPN) 75-72-69-69, Choi Ho-sung (KOR) 71-71-72-71