The 25-year-old centre back, who has 26 caps for his country and made four appearances at the World Cup where Argentina reached the final, signed a four-year deal with the Welsh club after passing his medical on Monday.
His move is subject to international clearance but the club say they expect the player to go straight into the squad for Saturday's league fixture at home to newly-promoted Burnley, following their opening day win at Manchester United last weekend.
Fernandez was part of the Napoli team that won the Coppa Italia last season and played in both of their Champions League and Europa League campaigns.
Injury meant he missed both legs when the Italian side knocked his new club out of the Europa League in the round of 32 in the previous campaign. (Reporting By Sam Holden)