The Swans stated on their website that Fernandez has signed subject to international clearance for fee reported to be around £7 million.
The 25-year-old passed a medical earlier this week, and is expected to be in Swansea's squad for their league game against Burnley at the Liberty Stadium on Saturday, when manager Garry Monk will look to build on last weekend's 2-1 win at Manchester United.
"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," Fernandez told the club's official website.
"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 (former Swansea loan player) 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."
Fernandez has won 26 caps for Argentina, including making four appearances at the World Cup in Brazil earlier this summer.
He joined Napoli in 2011 from Estudiantes for around £2.5million, helping them land the Coppa Italia last season during a campaign when they also finished third in Serie A.
Fernandez additionally featured in Napoli's Champions League and Europa League campaigns, although he was not involved in either leg of the Europa League last-32 tie against Swansea.
"I can identify myself with the Swansea style. I like to play the ball from the back and, of course, I love to defend too," Fernandez added.
"I was impressed last weekend with the team's performance and result against Manchester United.
"It was a very intelligent performance. The team really took its opportunities, and it was a fantastic start to the season.
"I just can't wait to start training and playing games."
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- Jordi Amat