The American continued her hot streak of form, winning by three shots from overnight leader Ai Miyazoto to bag her second title of the month.
She will jump from third to first in the new LPGA rankings, ending Yani Tseng's 109-week run at the top of the standings.
Lewis' blistering fourth round 64 proved too much for Miyazoto, who shot 71, to fend off. The crucial blow was struck at the 16th, where Lewis found a birdie and her Japanese opponent slumped to a double bogey.
''I'm having a blast on the golf course, and to be No. 1 in the world, it's what everybody out here on tour is working for and to be that person is, I mean, I really don't even know what to say,'' said Lewis.
''All in all, I had a good day,'' Miyazato added. ''I have a good feeling and my play was really solid, just the one bad shot on 16. I was a little disappointed.''