Saif al-Islam Gaddafi, second son of the late deposed Libyan leader Col Muammar Gaddafi, is said to have been freed under an amnesty, in a move which could fuel further instability.
The news of his release was made known by a statement made by the Abu Bakr al-Siddiq Battalion, a militia which controls Zintan.
The Abu Bakr al-Siddiq Battalion said he had been released on Friday but he has not been shown in public.
He was sentenced to death in July 2015 by a court in Tripoli in a mass trial of former Gaddafi government officials.
He is wanted by the International Criminal Court for crimes against humanity during his father’s unsuccessful attempts to put down the rebellion.
Since Gaddafi’s four-decade rule ended in 2011, Libya has struggled to establish basic institutions and rule of law, with militias and former fighters challenging the authority of the weak central government.