"Swansea City are delighted to confirm the signing of Napoli and Argentina defender Federico Fernandez for an undisclosed fee, subject to international clearance," read a statement on the Premier League club's website.
"The 25-year-old passed his medical on Monday before putting pen to paper on a four-year deal at the Liberty today (Wednesday)."
While the fee for Fernandez was undisclosed, British media reports said that it was around £8 million ($13.3 million, 10 million euros).
Fernandez said he had received recommendations about Swansea from several of the club's players, including Spanish forward Michu, who is currently on loan at Napoli.
"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 Swansea website.
"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."
He added: "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 started four games for Argentina in their run to the World Cup final, where they were beaten 1-0 by Germany after extra time. He has won 30 international caps, scoring two goals.
He is expected to partner Swansea captain Ashley Williams in central defence, following the sale of Spanish defender Chico Flores to Qatari side Lekhwiya last week.
Subject to international clearance, Fernandez could go into the squad for Swansea's league game at home to Burnley on Saturday.
Fernandez started his career with Argentine club Estudiantes, before moving to Napoli in July 2011 for around £2.5 million. He spent time on loan at Spanish side Getafe in 2013.