The former world number one won the inaugural edition in 2011 and made it another victory to remember by signing off with a four-day total of eight-under-par 280 at the Amata Spring Country Club.
Overnight leader Marcus Fraser of Australia and Germany's Martin Kaymer had their chances to force extra-time with Westwood but fell just short on the 72nd hole and settled for a share of second place on 281 total.
India's Anirban Lahiri did not get the win he wanted as he signed off with a 73 to share sixth place with Thailand's Thongchai Jaidee and American Jonathan Moore at the Thai showpiece.
The 27-year-old, however earned a well-deserved place at the 144th Open at St. Andrews in 2015 as one of the top four players who are not otherwise exempted from the world's oldest Major.
Westwood took delight with his Thailand Golf Championship win in 2014 as he had to overcome intense pressure to lift the prestigious title again.
"When I won the Thailand Golf Championship in 2011, I had a pretty big lead. But this win is obviously very special as it was a very tight day out there and I didn't get off to the best of starts and I had lots of work to do," said Westwood.
Starting the round two shots back of Fraser, Westwood seemed to have fallen out of the equation when he opened with two successive bogeys.
But the Englishman showed his mettle with a stunning comeback where he fired four straight birdies starting from the par-four sixth hole.
He continued his amazing birdie blitz by marking his card with more red numbers on holes 11, 14 and 15 to seal his second Thailand Golf Championship win.
"My caddy, Billy, told me there were opportunities to pick some shots up in the middle of the round after I made those two bogeys. I did that and didn't drop anymore shots and gave myself lots of chances," said Westwood.
"This is my last event of the year and it's nice to finish with a win and go into Christmas and start the New Year with confidence. I know what I've got to work on and I'll be doing that over the next six weeks before the season starts again," added the Englishman.
Fraser was left to rue another missed opportunity of a grandstand finish.
The Australian started off promisingly with birdies on four, six and seven. He dropped a shot on nine but responded well with another two birdies on 10 and 11.
However, a bogey-five on 12 followed by another costly bogey putt made under pressure on the closing 18 resigned Fraser to tied-second place.
"I gave a good fight and I'm happy with how I've played considering how I just got back to playing competitive golf recently," said Fraser, who missed eight months of action due to a wrist injury.
Lahiri also responded positively to a disappointing day by declaring his intentions to be back to challenge for top honours again.
"I'm obviously very disappointed with how I played today. There were lot of positives out there for me and I was struggling on my game today.
"At least for my consolation I earned a spot at The Open next year. That would put some balm on my wounds. I've had a great year and I have a lot to look forward to," said Lahiri.
Leading scores after round 4 of the Thailand Golf Championship being played at the par 72, 7488 Yards Amata Spring GcC course (am - denotes amateur):
280 - Lee WESTWOOD (ENG) 70-71-72-67.
281 - Martin KAYMER (GER) 71-72-70-68, Marcus FRASER (AUS) 69-72-70-70.
283 - Tommy FLEETWOOD (ENG) 71-69-73-70.
284 - Scott HEND (AUS) 70-74-71-69.
285 - Thongchai JAIDEE (THA) 72-71-74-68, Jonathan MOORE (USA) 71-71-72-71, Anirban LAHIRI (IND) 71-73-68-73.
286 - Sergio GARCIA (ESP) 71-75-71-69.
287 - Kiradech APHIBARNRAT (THA) 76-72-71-68, Paul PETERSON (USA) 71-72-75-69, Terry PILKADARIS (AUS) 77-69-72-69.
288 - Thanyakon KHRONGPHA (THA) 69-77-73-69, Bernd WIESBERGER (AUT) 76-74-68-70, Charlie WI (KOR) 74-72-71-71.
289 - Kodai ICHIHARA (JPN) 78-68-72-71, Daisuke KATAOKA (JPN) 74-73-71-71.
290 - Danny CHIA (MAS) 77-74-72-67, Berry HENSON (USA) 73-72-74-71, WANG Jeung-hun (KOR) 73-72-73-72, Unho PARK (AUS) 76-74-68-72.
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