The Lebanese army said the Sierra Leone-flagged “Lutfullah II” ship was carrying three containers of machine guns, artillery shells, rocket launchers, and other weapons.
The ship reportedly had sailed from Libya.
Syrian authorities have repeatedly alleged that weapons are being smuggled from Lebanon to rebels fighting the regime of President Bashar al-Assad.
In another development, the United Nations says the new head of its monitoring mission in Syria is on his way to Damascus.
UN spokesman Neeraj Singh told reporters that Major General Robert Mood, a veteran Norwegian peacekeeper, was expected to arrive on April 29 to serve as the world body's chief military observer in Syria.