Somboon Seawang, 23, was held by officers in Ranong, the Thai-Burma port north of Phuket, after a sting.
Caught when police tried to buy 15 packs of ya baheach containing 2000 tablets, a visit to Khun Somboon's room at the Ranong Gardens hotel revealed packs containing 400,000 tablets.
The drugs were destined for distribution in Ranong, and the neighboring provinces of Phang Nga and Phuket, officers said.
The haul was estimated to have a street value of 100 million baht. Police also took possession of three telephones, three bank books and an ATM card.