Hezbollah leader Hassan Nasrallah was killed in an Israeli airstrike on the southern outskirts of Beirut, Lebanon more than a month ago.
In a statement on October 29, Hezbollah said the group's Shura Council elected Qassem, 71, as the group's leader.
In 1991, Naim Qassem was appointed deputy leader of Hezbollah by then Hezbollah leader Abbas al-Musawi - who was killed in an Israeli helicopter attack in 1992.
Qassem continued to serve as Hezbollah's deputy leader when Nasrallah became leader and has long served as one of Hezbollah's top spokespeople, giving interviews to foreign media in recent times.
Leader Nasrallah was killed on September 27 and senior Hezbollah figure Hashem Safieddine - seen as the most potential successor - was also killed in Israeli air strikes a week later.
Since Nasrallah's death, deputy leader Qassem has made three televised speeches, including one on October 8 in which he said the militant group supported efforts to reach a ceasefire with Israel.
