Officers acting on a tip-off arrested Charnchai Kulthasri, 24, and Buatee Saisom, 31, were arrested in Phayao province while they were on a tour bus heading from Chiang Khong district of Chiang Rai to Ayutthaya.
Police discovered 200,000 speed pills wrapped in aluminium foil in the men's luggage, said Pol Gen Pongsapat Pongcharoen, the secretary-general of the Office of the Narcotics Control Board (ONCB).
After questioning the two suspects, police later arrested Boonlert Muang-ngern, 24, in Bang Pa-in district, Ayutthaya. Mr Boonlert was to have received the drugs from the men, officers said.
Police also seized a pickup truck and six mobile phones.
Mr Charnchai, who was the tour bus driver, confessed that he had been hired for 500,000 baht by a drug trafficker in Chiang Rai to deliver the pills to Mr Boonlert in Ayutthaya. He said he had asked Mr Buatee, the bus conductor, to prepare and hide the luggage, according to police.
The suspects told investigators that they had made many other drug deliveries in the past.
They were charged with possession of illegal drugs with the intent to sell.