Navy spokesman Captain Athula Senarath said the 90-metre (300-feet) long Princess Christina had been captured at a foreign port but refused to reveal the location or identify the country.
"We can't say from where we seized the ship, but we deployed our own crew and brought it to Sri Lankan territorial waters," he said.
Sri Lanka's navy last year claimed it had destroyed a fleet of 10 gun-running ships operated by the separatist Liberation Tigers of Tamil Eelam (LTTE) which was finally defeated in May after a 37-year conflict.
The group's new international representative, Selvarasa Pathmanathan, was taken into custody in August, three months after security forces claimed victory over the rebels.
Pathmanathan, better known as KP, the chief arms smuggler for the Tigers, is in military custody in Sri Lanka after he was arrested in Southeast Asia in August, but has yet to be brought before courts.
The government believes the LTTE sourced most of its weapons from Southeast Asia