Officials became suspicious about two unusually heavy bags left on the carousal at Suvarnabhumi Airport, according to a statement from the customs department.
After X-raying the suitcases, they found 423 radiated tortoises, as well as 52 unknown species of turtle or tortoise.
The statement said no suspect was arrested over the bags, which arrived in Thailand on a flight from Bangladesh.
Radiated tortoises are protected under the Convention on International Trade in Endangered Species (CITES).
Thailand, seen as a hub for traffickers of many endangered species, came under pressure over the rampant smuggling of ivory through its territory during CITES talks in Bangkok this year.