"This is the largest stock of LTTE pistol ammunition ever to be recovered by the security forces," navy spokesman Kosala Warnakulasuriya told AFP. "We think this ammunition was buried over five years ago."
Troops found 120,000 rounds of pistol rounds along with 8,000 rounds of other ammunition.
The navy also released photographs of the ammunition from what it said was the same area where the final civil war battles were fought five years ago.
The navy said the discovery was made on "information received" by them, but gave no details.The find comes amid government claims that remnants of the LTTE are trying to regroup and stage a comeback five years after their leader Velupillai Prabhakaran was killed by security forces, ending nearly four decades of ethnic war.
Human rights groups have accused Colombo of trying to whip up fears of a Tiger resurgence to justify draconian laws and maintain a large military presence in the former conflict zone.
The military last month shot dead three Tamil men accused of trying to revive the LTTE in the island's north.
Sri Lanka's ethnic war between 1972 and 2009 claimed at least 100,000 lives, according to UN estimates.
The Tigers, who during the height of their power controlled nearly a third of Sri Lanka's territory, were known for their trademark suicide bombings, which claimed high-profile targets, including a president in May 1993.