Choi and Hjorth played in the same group and were tied heading to 18, where they both hit nice tee shots to the left side of the fairway. Hjorth hit a 4-iron into a water hazzard, while Choi laid up in front of the green.
The Swede manage to save par, while Choi chipped within inches for the tap-in birdie.
She finished at 10-under 278 at Sky 72 Golf Club's Ocean Course, where the LPGA was making its annual tour stop in South Korea. Choi won the Samsung World Championships in September in similar fashion, with an eagle at the par-5 last hole.
"This time I was able to play the last hole comfortably," Choi said. "I went in thinking birdie."
Tseng, who contended with the leaders most of the round, also made birdie on the final hole to claim a share of second place.
Ran Hong, who mainly plays on the Korean LPGA Tour, shot a 3-under 69 to finish four strokes off the lead, one ahead of Song-Hee Kim (71). Jiyai Shin of South Korea finished with a 70 to finish in alone in sixth, done in by a 6-over 78 in the third round.
American Brittany Lang, Anna Nordqvist of Sweden, and South Koreans Se Ri Pak and Inbee Park rounded finished at 2 under 286. Top-ranked Lorena Ochoa closed with a final-round 73 to end at 6-over, tied for 44th.
Choi donned traditional Korean attire during her trophy presentation, as is customary for winners of the tournament.
"I take pride in winning for Korea this week," Choi said.
The LPGA Tour swings to Japan and Mexico before closing with the inaugural Tour Championship in Houston.