Napoli confirmed the deal in an official club statement: "SSC Napoli announces the exercise of the option for the definitive acquisition of the sporting services of the player Alisson de Almeida Santos from Sporting Clube de Portugal."
Alisson joined on loan in January and made an immediate impact. He debuted on February 10 in a Coppa Italia tie against Como, scoring in a penalty shootout.
Five days later he scored in a Serie A match against Roma at the Maradona, helping Napoli to a 2-2 draw.
He went on to make 15 competitive appearances in total, scoring four goals and producing a series of high-level performances that convinced the club to trigger the purchase clause without hesitation.
The permanent deal follows the confirmed signing of Rasmus Højlund from Manchester United — also triggered by a pre-agreed obligation this month — making Santos the second acquisition Napoli have formalised this summer as head coach Massimiliano Allegri begins assembling the squad he will work with.