Rohitha Bogollagama said the Liberation Tigers of Tamil Eelam (LTTE) were one of the few "terrorist organisations" in the world with a proven maritime capability.
Along with their Sea Tigers naval combat unit, they owned a substantial number of ships which sailed under the flags of different countries and which, alongside traditional trading, smuggled military hardware, Bogollagama told the Shangri-La Dialogue of Asian defence and security officials and experts.
There have been stepped-up clashes between suspected rebel ships and the Sri Lankan navy in recent months amid heavy fighting in the island's northern and eastern regions.
Bogollagama said the Tigers "have carried out numerous acts of maritime terrorism" over the years.
"This underlines (that) the LTTE's capacity to be a threat to international shipping is clear," he told delegates.
Last month, 11 southern Indian fishermen who went missing in March returned home alleging they were abducted at sea and kept captive by