The world number two carded six birdies in a four-under-par 68 to finish on 21 under par at Hoakalei Country Club on Oahu.
South Korea's Yoon In-a was second at 18 under after also finishing with a four-under-par 68.
Thai star Thitikul suffered a scare coming down the stretch with a double-bogey six on the 16th that dropped her to 19 under.
But back-to-back birdies on 17 and 18 restored her cushion and averted the possibility of a play-off.
"I totally have no idea how I did that," Thitikul said of her birdie-birdie finish. "It was a really good battle with In-a. She played incredible on the front nine.
"I knew I had to focus on my game. The 16th was terrible, a terrible hole for me. But I just tell myself that it's two holes left and everything can happen."
Japan's Miyu Yamashita finished in third place on 17 under after her final-round 67, one shot ahead of fourth-placed Auston Kim of the United States.
China's Michelle Zhang and Sweden's Maja Stark shared fifth place at 15 under, six off the lead.