Israel said it killed Ali Yusuf Harshi, nephew and personal advisor to Hezbollah's Naim Qassem, in an overnight strike in Beirut.
The Israeli Defence Forces (IDF) said Harshi played a central role in managing Qassem's office and was a close associate.
Israel also struck two key crossings used by Hezbollah, 10 weapons storage facilities, and command centres in southern Lebanon.
The strikes come despite a two-week ceasefire between Iran and the US, with Israel saying the ceasefire does not apply to Beirut.
Over 1,500 people have been killed in Israeli strikes across Lebanon since March 2, including 130 children and over 100 women.
