The 25-year-old ran away with victory via a crucial birdie that she managed to sink from an awkward spot on the green, before taking a even par on the final hole to card a two-under 70 on the last day of the season-ending tournament.
That birdie followed two earlier and an eagle on the par-five fifth hole as she regained composure after a double-bogey at the third.
The win is Choi’s second this year and supplements an outstanding 2012 LPGA campaign during which she also won the US Women's Open. The victory was worth $500,000 and took her total prize money for the season to $1.9 million.
"I'm really happy with how I played this season," Choi said.
"I won my first major and even this tournament is very big for me. I think I can have even bigger expectations now and think I deserve it."
Ryu was tied for the lead with Choi for the first 13 holes, until a three-putt bogey on the 14th relegated her to second, the position she stayed for the rest of the round.
The Rookie of the Year award winner finished with a 70 for 276.
Meanwhile, South Korea's Inbee Park finished off with a 70, which was enough to see her wrap up the Vare Trophy for the lowest scoring average.
Stacy Lewis finished down in equal 29th but had already became the first American since 1994 to win the LPGA Player of the Year award.