BEIJING, 1 Nov 2015, (AFP) - The 22-year old shot a final-round 70 to finish on two-under-par 286.
That score put her one shot clear of Stacy Lewis of the US, Taiwanese Candie Kung and Kim Kaufman, who tied for second place.
The win was Kim's third LPGA Tour victory. She trails only Lydia Ko and Inbee Park in victories this season on the Tour.
The South Korean wins $300,000 in prize money.
"Yeah, I was really nervous," said Kim in comments on the LPGA website. "I just focus, and when I hit the putt, I couldn’t believe it."
Number one ranked Lydia Ko shot a final-round 70 to finish on 292 and move up from 20th position to joint eighth.
Final round scores from the Blue Bay LPGA tournament at par-72 Jian Lake Blue Bay Golf Course in Hainan, China on Sunday:
286 - Kim Sei Young (KOR) 70-72-74-70
287 - Kim Kaufman (USA) 72-72-74-69, Stacy Lewis (USA) 71-73-73-70, Candie Kung (TPE) 71-72-73-71
291 - Sandra Gal (GER) 73-73-74-71, Xi Yu Lin (CHN) 67-77-75-72, Alena Sharp (CAN) 74-70-74-73
292 - Lydia Ko (NZL) 77-75-70-70, Ariya Jutanugarn (THA) 74-73-75-70, Ilhee Lee (KOR) 72-75-74-71, Jane Park (USA) 70-74-77-71, Suzann Pettersen (NOR) 74-69-75-74