The win was Yi's first on the LPGA Tour.
Yi carded an even-par 71 in the final round of regulation play and Pressel shot a 67, tying at 18-under 266.
The playoff began at No. 18, where Pressel missed a 25-foot birdie attempt before Yi won it with her birdie.
Michelle Wie (64), Seon-Hwa Lee (67) and Song-Hee Kim (69) tied for third place at 16-under 268.
The 21-year-old Yi, who lives in Murrieta, Calif., is a native of South Korea. She had not finished in the top 10 in any prior event this season.