Westwood carded a three-under 69 in the rare Saturday finale for a four-round score of 269, five shots ahead of Ross Fisher, who shot 71 for the day.
Saturday capped a sizzling tournament for Westwood, who was the only player in the field to break 70 in all four rounds.
"The positive is that I played really well this week," said Westwood, who turned in a 64 on Thursday sandwiched between two rounds of 68. "You don't win tournaments by five shots without playing really well."
Westwood played a relatively steady round with one bogey offset by a pair of birdies and an eagle on No. 12.