The tribunal pronounced the verdict Tuesday against Abdul Quader Mollah in a packed courtroom at the High Court premises in Dhaka.
Mollah was indicted last year on six war crimes charges, including mass murder, conspiracy and instigation in 1971. A former party member was sentenced to death last month.
The government of Prime Minister Sheikh Hasina in 2010 initiated the process of trying those accused of committing crimes against humanity during the war, under an amended 1973 law.
Jamaat-e-Islami—a key partner in a former Bangladeshi government—says the charges are politically motivated. Authorities deny the claim.