Officers arrested the pilot on March 6th after he allegedly landed a helicopter loaded with 750 kilos (1,650 pounds) near Estepona in southwestern Spain.
“The pilot was carrying out a sentence at a correctional centre and took advantage of his weekend furloughs to carry out the drug flights,” said a police statement.
The pilot was serving a sentence for an undisclosed crime at a jail in the seaside city of San Sebastian in Spain's northern Basque Country.
Police seized 1.5 tonnes of hashish as part of the operation along with 1,000 marijuana plants, two helicopters, 15 vehicles and €220,000 ($245,695) in cash.
Conveniently located at the crossroads of North Africa and the rest of Europe, Spain is a major transit point for drugs entering the continent.
Spanish police seized around 100 tonnes of hashish last year, about three percent less than in the previous year.