The craft was found on Saturday submerged about 500 metres (yards) off the coast of Vellamullivaikal in the district of Mullaittivu, the army said in a statement.
"Based on information provided by an informant...
the submersible was found and brought ashore with the help of army divers," the statement said.
Sri Lanka's navy has already begun investigations into the naval capability of the defeated Tamil Tiger rebels to establish how they acquired the technology and supplies, a military official said.
Several submersible devices had been found by the military in the run up to the final defeat of the Tamil Tigers on May 18 with the killing of the entire Tiger leadership at Vellamullivaikal.
The Tigers had a sea going unit known as the Sea Tigers and had sunk several naval craft as well as merchant ships off the island's northeast coast.
Last week, troops uncovered one of their biggest hauls of weapons and explosives since the defeat of the rebels.
Troops found 332 r