Pungluang Sor Singyu lost his WBO bantamweight title after being stopped by Marlon Tapales of the Philippines in Ayutthaya yesterday.
In the second defence of his 118-pound belt, Pungluang was knocked out in the 11th round in a mandatory fight.
The Thai would have won if the bout had last the 12-round distance as he was leading.
In fact, he could have finished off Tapales in the fifth round when he knocked down the challenger twice.
The Filipino then staged a rally to punish the Thai who was knocked down in the sixth.
In the 11th round, Tapales threw everything at Pungluang who was floored with a thunderous left punch to his chin.
Tapales' record improved to 29 wins (12KOs) and two losses.
It was Pungluang's fourth loss against 52 wins (35KOs).
With his defeat, Thailand now has only two world champions -- WBC mininmumweight title-holder Wanheng Menayothin and WBA minimumweight king Knockout CP Freshmart.
In another development, Wanheng, 30, will fight challenger Saul Juarez of Mexico in Chon Buri on Aug 2.
It will be Wanheng's fifth defence of the title after beating Mexico's Oswaldo Novoa, also in Chon Buri, in 2014.
Meanwhile, WBC bantamweight champion Shinsuke Yamanaka will attempt to defend his title for an 11th time on Sept 16 against Anselmo Moreno in Osaka, Yamanaka's Teiken Gym announced.
Hozumi Hasegawa will take a shot at the WBC super-bantamweight crown held by Hugo Ruiz on the same card in a world doubleheader at Edion Arena.
The 33-year-old Yamanaka narrowly edged Moreno in a split decision in September, and the hard-hitting southpaw wants to leave nothing in doubt this time against the Panamanian.
"I want to settle the score once and for all," said Yamanaka.