A Hezbollah statement did not say when the group's top officials voted to re-elect Nasrallah, who has held the post since an Israeli helicopter gunship killed his predecessor, Sheik Abbas Musawi.
No one ran against Nasrallah.
Naim Kassim also has been re-elected as Nasrallah's deputy, according to the statement.
Hezbollah's leadership elections are usually held every three years but they were last held in 2004. A Hezbollah official said the two-year delay was caused by internal Lebanese differences and the 2006 Hezbollah-Israel war. The official spoke on condition of anonymity because he is not authorized to speak to the media.
Nasrallah, 49, is a strong critic of the United States which lists his group as a terrorist organization. Under his leadership, Hezbollah became active in internal Lebanese politics and the group now has two Cabinet ministers and holds 11 of parliament's 128 seats.
Founded in 1982, Hezbollah gained notoriety in the West when it was linked to the kidnapping of Western hostages and a series of bombings against Western targets.
Hezbollah has over the years grown to run a network of interests — clinics, schools, a TV station and a weekly newspaper. It caters mainly to Lebanon's 1.2 million Shiite Muslims, the country's largest single sect.