Almost 29 billion baht worth of contraband ketamine, most of it probably destined for Europe, Japan or Korea, were seized, the Narcotics Suppression Bureau said on Thursday.
Montri Yimyaem, head of the bureau, said the size of the haul pointed to a multinational network, and that Thailand would pursue it with other countries and international agencies.
"The drug is not popular in Thailand, and is mostly sold abroad, including Europe, Australia, Taiwan, Japan and Korea," he said.
In medicine, ketamine is used as an anaesthetic or an anti-depressant, but as a recreational drug it is used to induce dreamy or trance-like sensations, and sometimes hallucinations.
Investigators found 457 25kg bags of the drug, or 11.4 tonnes in total with market value of 28.7 billion baht, at a warehouse in Chachoengsao province, east of Bangkok.
They had expanded their investigation after 300kg of ketamine that had come from Thailand was found in Taiwan.
"This discovery is only the beginning, there will be further investigations to seize ill-gotten gains connected to drug smuggling," said Minister of Justice Somsak Thepsutin.