The former world number one, winner of the event on his two previous appearances in 2011 and 2012, could only muster a final round of one-over-par 73 and seven-under total of 281 after letting slip the healthy overnight advantage.
He had the opportunity to register victory on the par-five 18th only to fail with a birdie effort, meaning a playoff with Chapchai, who produced a sparkling 66 after snapping an iron against a tree early in Saturday's third round.
The Englishman made no mistake on the 18th in the playoff, however, notching a birdie on the first extra hole to edge out the four-times Asian Tour winner Chapchai.
“I was lucky that I already had a read on (my putt on) 18 for the playoff,” Westwood said.
“It was a tough day out there.. but managed to battle back.
“Its nice to have won here three times, its a good hat-trick.”
Westwood had started the day seven clear of Chapchai at the Royal Jakarta Golf Club after covering his back nine of the third round in just 29 shots, but gave encouragement to the chasing back with two bogey fours on his opening nine.
He seemed to find his game again with three birdies in four holes, including a long, snaking downhill putt on the par-four 11th to start his back nine but back-to-back bogeys on 16 and 17 dropped him level with Chapchai.
Australian Kalem Richardson finished third after a strong 66 left him at six-under, with Dane Thomas Bjorn (70) a further shot back in fourth and former US PGA Champion Yang Yong-eun (71) in a share of fifth. – Reuters
Original source: Westwood makes it a hat-trick