The special court responsible for terrorism cases, ordered the conditional release of Said Ben Iazza, who has already served more than half of his eight-year sentence.
Iazza lent a vehicle and documents to the men who carried out the attacks.
Although paroled, he has been banned from leaving Spanish territory and had to hand over his passport.
Iazza was only convicted in May of this year, along with two others jailed for 46 and 53 years.
The attacks, which also injured 140 people, were carried out by a cell of young people of Moroccan descent who grew up in Catalonia, but claimed by the Islamic State group.
On August 17th, 2017, a 22-year-old drove a van down Barcelona’s Las Ramblas avenue at high speed running over pedestrians before fleeing in a car he stole after killing the driver. He was shot dead by police four days later.
Five of his accomplices drove into more pedestrians and stabbed a woman who later died of her injuries in Cambrils, a seaside resort 100 kilometres (60 miles) away. Police shot dead all five.