The former world No 1 moved into contention on day two but was still two shots behind overnight leader Marcus Fraser of Australia on Saturday in Bangkok.
But the 41-year-old Englishman, who won the tournament by seven strokes from Charl Schwartzel in its debut season in 2011, put in a five-under-par round to finish eight under overall, one shot ahead of both Ryder Cup team-mate Martin Kaymer and Fraser.
Westwood was delighted with his victory and tweeted afterwards: “It’s been a good last day of school” – a nod to his season now being at a close.
Southport’s Tommy Fleetwood was two shots further back in fourth after carding a final-round 70 at Amata Spring Country Club, while defending champion Sergio Garcia finished ninth on two under after his score of 69 yesterday saw him move up a spot.
Bubba Watson of America ended the tournament in joint 25th after struggling with scores of 76 and 77 in rounds one and three respectively – on the par-72 course – while Northern Ireland’s Darren Clarke fared even worse with scores of 75, 75, 73 and 74 meaning he finished in a five-way tie for 50th.
Greg Chalmers won the Australian PGA Championship for the second time after holding firm in a gruelling play-off.
Chalmers, Adam Scott and Wade Ormsby all ended 72 holes tied on 11 under par, so headed off to play the 18th once more.
In the case of Chalmers and Scott – Ormsby dropped out after three – that turned into a seven-hole shootout. Chalmers, 41, stuck with it and when Scott missed a four-footer for par, rolled in for the win, to add to his 2011 success in the event.