Kriangkrai Techamong stole the precious gems from the palace of a Saudi prince where he worked in 1989, triggering a feud between Thailand and Saudi Arabia dubbed the "Blue Diamond Affair" that has yet to be resolved.
Thai police later returned some of the jewels but Saudi officials claimed most were counterfeits while the whereabouts of the most precious gem -- a rare 50-carat blue diamond -- remains unknown.
Today, Kriangkrai told local media his life has been haunted by the theft that unleashed an "avalanche" of suffering on his family.
"I am confident that all my misfortunes are the result of a curse from the (blue) Saudi diamond I stole, so I've decided to enter the monkhood for the rest of my life to redeem my bad karma," he told Thai Rath newspaper.
Local TV channels showed the middle-aged man receive alms from temple-goers as he marched in an ordination ceremony with a shaved head and white robes in northern Lampang province.
Channel 7 reported that he had been blessed with a new monk name that translates to 'He Who Has Diamond Knowledge'.
Kriangkrai was jailed for five years soon after the theft, but managed to sell most of the gems before his arrest.
Saudi Arabia has long accused Thai police of bungling its investigation, with widespread allegations at the time that the stolen items were snapped up by senior officers.
Riyadh sent a businessman to conduct his own investigation, but he disappeared in Bangkok days after three Saudi diplomats were shot dead, execution style, in the city.
In 2014 a case was dropped against five men, including a senior Thai policeman, for alleged involvement in the businessman's murder over lack of evidence. The decision came after a last-minute change in judge.
Saudi Arabia has not sent an ambassador to Thailand for decades and restricts travel between the two countries because of the unresolved theft and murders.
(This story has not been edited by NDTV staff and is auto-generated from a syndicated feed.)
Get Breaking news, live coverage, and Latest News from India and around the world on NDTV.com. Catch all the Live TV action on NDTV 24x7 and NDTV India. Like us on Facebook or follow us on Twitter and Instagram for latest news and live news updates.