The Pentagon said Said bin Brahim bin Umran Bakush was transferred to Algeria after an official decision on his release was made earlier this year.
Detaining Bakush, 52, was deemed "no longer necessary to protect against a continuing significant threat to the national security of the United States," the Pentagon said in a statement.
Bakush was apprehended in 2002 in Faislalabad, Pakistan as the US swept up hundreds of suspected Al-Qaeda operatives and fighters in the wake of the September 11, 2001 attack on the United States by the group.
Although never seen as more than a low-level Al-Qaeda fighter not directly connected to the 9/11 plot, he was nevertheless held since then at the prison on the US base at Guantanamo Bay, Cuba.
Like fellow prisoners, he was deemed an enemy combatant without recourse to the US justice system.
With Bakush's release, 30 detainees remain at Guantanamo, down from a peak of nearly 800.
Of them 16 are eligible for transfer and the Pentagon and State Department are seeking countries to accept them.
Another three are eligible for a Periodic Review Board assessment, while nine are facing charges under military commissions and two have been convicted in such commissions.
Algerian Freed From US Guantanamo Custody
WASHINGTON - The US military said Thursday that it had released an Algerian held at the Guantanamo prison for two decades, leaving 30 men still held extrajudicially at the US navy base in Cuba.