Santiago Arrospide Sarasola, better known as Santi Potros, was sentenced to a total of almost 3,000 years in prison, but in Spain actual jail time cannot exceed 30 years.
He was found guilty of involvement in a number of attacks, including Eta's most deadliest, the 1987 bombing of a Barcelona supermarket that killed 21 people.
He was arrested in France the same year but not extradited to Spain until 2000.
While in prison he distanced himself from some of Eta's tactics, criticising a 2006 car bomb at Madrid's Barajas airport in which two people died.
At least one victims' group has criticised his release.
Eta announced its own dissolution in May.
Its attacks claimed 853 lives, according to the Spanish government.