Seizure was made at Mandapam near here after the police saw some persons getting down from the van and running away, leaving the vehicle, said Sub inspector Marine Group Jothi Basu.
The value of the seized sea cucumber, an endangered marine species, would come around Rs one crore in the international market, Basu said.
A search was on to nab those who were in the van, he said.
Trade of sea cucumber is prohibited in India.