The world number three secured his 38th career victory after he returned early in the morning to complete his third round before closing with a final round two-over-par 74 at the Royale Jakarta Golf Club.
Englishman Westwood's winning total of 16-under-par 272 meant he finished two shots ahead of Thailand's Thaworn Wiratchant, with the Indian duo of Gaganjeet Bhullar and Shiv Kapur tied in third place.
The 38-year-old, who finished tied third at the Masters two weeks ago, held an eight-shot lead going into the final round but tiredness began to kick in on his homeward nine as he shot three bogeys.
Play at the Asian Tour event had been abandoned on Saturday due to lightning.
"It was draining out there and I was really feeling it on the back nine. When you are in contention and under pressure, it seems to zap your energy faster," said Westwood.
"You never know how to play with such a big lead. You don't know if you should attack or defend. It is quite hard to get your mental attitude around it.
"It was a case of staying patient and calm and playing the holes correctly. It was exhausting."
Westwood, who collected a winner's cheque of $118,875, said he hoped the win would kick-start this year's attempt at winning an elusive Major championship.
"I've played well without any wins so hopefully I can go on and win other tournaments.
"It is tough when you are the favourite because if you don't win then it will be disappointing. There was a lot of pressure so I'm glad I did it."
Thaworn, winner of the Asian Tour Order of Merit in 2005, birdied the last two holes to claim his best finish of the season.
"I felt tired after the 12th hole but I held on. I couldn't really hit my shots properly so I'm quite surprised with the way I finished.
"I haven't won in a while but I've come close this season so I hope to keep the momentum going."
Leading final scores (par 72)
272 - Lee Westwood (ENG) 65-68-65-74
274 - Thaworn Wiratchant (THA) 72-69-66-67
276 - Gaganjeet Bhullar (IND) 72-70-65-69, Shiv Kapur(IND) 68-71-70-67
277 - Anirban Lahiri (IND) 74-68-66-69
278 - Seuk-hyun Baek (KOR) 72-70-68-68
279 - Angelo Que (PHI) 69-73-68-69, Lee Sung (KOR) 68-71-67-73, Bernd Wiesberger (AUT) 70-68-72-69
281 - Berry Henson (USA) 69-73-69-70, Wade Ormsby (AUS) 71-73-66-71, Masanori Kobayashi(JPN) 69-68-74-70, Zaw Moe (MYN) 68-67-75-71