Westwood finished 72 holes at 22-under 266, well ahead of reigning Masters champ Charl Schwartzel, who shot a 15-under 273. Westwood finished the tournament with a three-under 69, while Schwartzel shot an even-par 72.
It's the 37th career win for Westwood, and he'll move into second in the Official World Golf Ranking on Monday ahead of Rory McIlroy.
"It means a lot," Westwood said. "Any win does. It's special this week because of the difficulty of the golf course and whom I was up against in the weekend."
Behind Westwood and Schwartzel was American Michael Thompson (70) at 14-under 274, while Simon Dyson (69) and Chawalit Plaphol (67) shared fourth at minus-12.
"If it wasn't for Lee, I would have probably won the tournament," Schwartzel said.
Guido Van Der Valk (73) and Daisuke Maruyama (70) tied for sixth at nine- under.