The 247 tusks, some up to 6.5 feet (2 meters) long, were found Wednesday during an X-ray scan of a shipping container labeled as frozen mackerel, customs officisals said. They valued the tusks at 100 million baht ($3.3 million).
"This is the biggest lot that we have ever seen," said Prasong Poontaneat, the Customs Department director-general. "By looking at the length and appearances, they could not simply cut the ivory off, but had to kill the creatures to get these tusks."
Poaching of elephants in central and eastern Africa has intensified in recent years, with much of the illegal ivory exported to Asia.
Ivory shipped to Thailand typically goes to carvers who fashion it into Buddhist statues, bangles and jewelry for sale to tourists or for export to other countries. Thailand is also a transit point for ivory forwarded to other markets like China.
Earlier this week, police raided illegal ivory carving workshops in central Thailand and seized large pieces of African elephant tusks and 6.6 pounds (3 kilograms) of hippopotamus teeth and walrus tusks.
The shipment seized Wednesday originated in Kenya and was headed for a shipping company in Bangkok's suburbs, Prasong said. He said investigators found the company closed and were still searching for the intended recipients.
Thai and Kenyan authorities have worked together over the past year to investigate and crack down on illegal ivory trafficking, which threatens the survival of elephants and other rare wildlife, according to FREELAND, a Bangkok-based organization that fights wildlife and human trafficking.
An increase of seizures at Bangkok's Suvarnabhumi International Airport has prompted traffickers to send their hauls by ship, said Doug Goessman of FREELAND.
Thailand's previous largest ivory seizure was a load of 239 African elephant tusks weighing about 2 tons that was confiscated at the Bangkok airport in February 2010.
Under Thailand's Wild Animal Reservation and Preservation Act, possession of elephant tusks without a permit is punishable by up to four years in jail or a fine up to 40,000 baht ($1,330) or both.
