The deal for the 25-year-old World Cup runner-up is undisclosed but is understood to be about £8m.
The type of players that we try and identify to come in here are ones that add quality to the squad and he definitely will do that
Fernandez joins on a four-year contract and should be included in Swansea's squad to face Burnley on Saturday subject to international clearance.
He started four of Argentina's games during their run to the World Cup final.
Fernandez comes in after manager Garry Monk moved to replace 27-year-old Chico Flores following his switch to Qatar.
The centre-back revealed his delight in joining the Swans after getting advice from Michu.
"It has always been my dream to play in the Premier League - it is the best in the world - and I'm grateful to Swansea for giving me this opportunity," he said.
"I asked Michu and Jordi Amat about the club. I obviously trained with Michu at Napoli and while I was on loan at Getafe [in 2013] I played with Alvaro Vazquez. He is very good friends with Jordi, so I've spoken to them too.
"Michu told me it is a family club and the city is very relaxed. He said I was joining a very good club. All I've heard are good things about Swansea."
The Swans play their first home Premier League match against Burnley on Saturday, 23 August following their 2-1 win away at Manchester United in their opener.
Fernandez has only recently returned to training following his World Cup exploits so it is not known whether he will be ready for the match.
Monk said the defender, who made 13 appearances on loan at Spanish club Getafe last season, is a good acquisition for the club.
"He brings that pedigree where he's been playing for Napoli at the top level and for Argentina at the World Cup.
"The age that he's at is very good for us and the type of player that he is.
"He's a good size, a very good footballer and someone who will suit Swansea City.
"The type of players that we try and identify to come in here are ones that add quality to the squad and he definitely will do that."
Swansea City have completed the signing of Argentina defender Federico Fernandez from Italian side Napoli.