The deal – which is subject to international clearance – was completed after Fernandez, 25, passed a medical on Monday.
Fernandez, who will wear the number 33 shirt, will compete with captain Ashley Williams, Jordi Amat and Kyle Bartley for a place in the heart of the Swansea defence.
He is expected to go straight into the squad for Saturday’s home game against Burnley.
Speaking last week, Swansea boss Garry Monk was full of praise for Fernandez, saying: “He is at a good age for us and will suit Swansea City.
"We want players who add quality to the squad and he will."
Fernandez helped Napoli win the Coppa Italia last season - they also finished third in Serie A - and made four appearances for Argentina at the World Cup in Brazil.He began his career with Estudiantes before joining Napoli in July 2011 - he has also spent time on loan at Spanish club Getafe.