The Englishman approached the final day at the Kuala Lumpur Golf and Country Club leading compatriot Andy Sullivan by one.
As his nearest rival faltered badly with a round of 78, former world number one Westwood maintained his form either side of a delay for thunderstorms, carding 68 to finish seven shots clear of Bernd Wiesberger, Louis Oosthuizen and Nicolas Colsaerts.
The victory marks Westwood's 13th career triumph in Asia and his second at the Malaysian Open, a tournament he previously won back in 1997.
In addition, the 40-year-old can take heart from a first tournament win in almost two years, with his last success having come at the Scandinavian Masters in June 2012.
Westwood, who finished seventh at the Masters last weekend, was faultless in his final round and moved into a four-shot lead before play was suspended due to the threat of lightning.
He was not affected by the unscheduled interval, moving six clear with a birdie at the 13th before capping an excellent victory by sinking a 15-foot putt to gain his fourth shot of the day at the last.
A dreadful round saw Sullivan's hopes of success quickly diminish, with a triple bogey at the second setting the tone for a disappointing day.
Sullivan responded to that early setback with consecutive birdes on holes four, five and six, but soon fell away again and finished in a tie for 13th at seven under.
By contrast, Austrian Wiesberger surged up the leaderboard with a round of 67, his only error a bogey at the 15th.
Oosthuizen and Colsaerts posted respective scores of 68 and 70 to join Wiesberger at 11 under, but it was Westwood who rightly took the acclaim for a superb display.
"I haven't won for a long time so it's nice to be back in the winners' circle," Westwood said. "I've just started playing well recently and obviously I'm delighted to have played well this week and won.
"I feel like I'm starting to swing it well, particularly today under pressure. After being four or five clear yesterday and it getting back to one, and then today, pulling away, I think that's a very important sign.
"I think a win is important at any time. Wins only give you confidence.
"I'm looking forward to playing the next tournament, which will be at Quail Hollow, the Wells Fargo, and then the Players Championship so it's a good time of the year to be playing well, there's a lot of big tournaments coming up."