The 739 pieces of tusk were found in a container which arrived at the port here on April 18 from the Democratic Republic of Congo and was heading towards neighbouring Laos, a statement by the customs said.
"The pieces weigh around 4,000 kilograms and are worth around six million dollars," the statement said, noting that it was the biggest ivory seizure in Thai history.
The container had been marked as carrying beans.
Registered ivory from domesticated Thai elephants can be sold in the country under Thai law, but wild life experts say that loophole allows criminals to smuggle poached African ivory through the country.
Thailand has stepped up seizure of illegal ivory after Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) threatened an international ban on the kingdom's entire wildlife business if it failed to curb the trade in tusks in the country by August 2015.