Lahiri, who was one shot ahead of Wiratchant going into the final round today, carded a two-under 70 for a four-day total of 270 to settle for a share of second place with Australia's Andrew Dodt and miss out on a third Asian Tour title of the season.
A win today would have meant more world ranking points for Lahiri and allowed him to close the gap on Order of Merit leader David Lipsky of the United States.
"I just missed way too many putts and it was disappointing to miss a short one on the last and that is going to hurt my world rankings. I'm upset with that. I just got to pick myself up and win next week," said the 27-year-old Indian.
Wiratchant rewrote the record books by winning an unprecedented 18th Asian Tour title after he closed with a five-under-par 67 at the Singha Park Khon Kaen Golf Club.
Qualifying School graduate Kalem Richardson of Australia enjoyed another good outing in Thailand by taking a share of fourth place with Thailand's Donthai Boonma and Paul Paterson of the United States.
Trailing overnight leader Lahiri by one shot, Wiratchant set off with a blistering pace by reaching the turn in 32. He picked up another advantage with a birdie on 10 before dropping his only shot of the day on the par-four 12.
