The haul of high-grade meth, better known as ice, has a street value of around USD 40 million, the Narcotics Suppression Bureau (NSB) said in a statement.
Seven Thais have been charged with drug trafficking, it added.
Wednesday night's bust unfolded after a car stopped suspiciously short of a police patrol in southern Chumphon province.
A police search found nothing in the vehicle.
"But the driver was acting suspiciously, refusing to answer mobile phone calls in presence of police... He began to shake and sweat," the NSB said.
More From This Section
The 18-wheel vehicle was found parked at a petrol station in Chumphon town with 500 bars of the drug wrapped in black plastic bags and hidden under sacks of corn husks, the NSB said.
"It's a record seizure of these drugs (ice)," Major General Dusadee Choosangkij told AFP.
"I believe they were to be stored in Malaysia and then destined for either Australia or Taiwan."
With long land borders, Thailand is a key route for ice, "yaba" -- the pill version of meth -- and heroin produced in factories in Myanmar and Laos.
Much of the production is controlled by Myanmar's powerful and heavily armed Wa ethnic group, whose high quality pills and heroin are stamped with their own logos.
The drugs are transported by car or foot across remote borders and sold to Thai gangsters.
Last week six Wa tribesmen were killed in a late night shoot-out with a Thai army patrol near the rugged Golden Triangle border region.