Lewis had an eagle on the par-5 seventh hole that allowed her to withstand a pair of late bogeys in shooting 1-under-par 71 Sunday for a tournament total of 15-under 273.
That was one stroke better than Choi Na-Yeon, whose even-par 72 Sunday left her at 14-under overall. Paula Creamer, with a 71 Sunday, took third at 13-under and Ariya Jutanugarn, who also was 1-under Sunday, was fourth with a 10-under tournament total.
Sunday marked the sixth LPGA title as a professional for Lewis. Five of those have come since last April.
Lewis and Choi opened the final round tied but Lewis was quickly up by two Sunday when she had a birdie at the par-3 second and Choi bogeyed the hole. Lewis gave back one stroke at the fifth hole and both were 14-under when Choi had a birdie at No. 6.
Choi also birdied No. 7 but Lewis, who just missed holing her second shot, tapped in for eagle for the lead. She pushed her advantage to two strokes with a birdie at No. 9 and was up three when Choi had a bogey at 13.
That was enough of a cushion for Lewis to survive bogeys at 15 and 17. Choi had a chance to force a playoff with a birdie putt at 18 but missed.