Hezbollah commander Hassan Lakkis killed in Beirut
Hassan Lakkis was "assassinated" near his home in Hadath - 7km (4.3 miles) south-east of the Lebanese capital, the BBC reported, citing Hezbollah TV channel Al-Manar.
Hezbollah blamed Israel for his death but Israel has denied the accusation.
Little is known publicly about Lakkis, but he was reputedly close to Hezbollah leader Hassan Nasrallah and an expert in weapons manufacturing.
The news comes a day after Hassan Nasrallah said Saudi Arabia was behind last month's bombings outside the Iranian embassy in Beirut.
Iran is a major backer of Hezbollah, which has sent fighters to Syria to back the government of Bashar al-Assad.
The conflict in Syria has increased sectarian tensions in its smaller neighbour.
However, the BBC's Jim Muir, in Beirut, says Lakkis might be an unlikely target for Sunni militants angered by Hezbollah's role in Syria.