"It was a long day but I’m really happy to be here," the centre-back said after signing a two-year deal at St James' Park.
"It’s a new challenge in my life and I’m very excited to meet all the players and staff here."
Fernandez will take the number 18 shirt for Newcastle, and Rafa Benitez pointed to his previous experience of working with the player.
"I know Federico well from our time together at Napoli and it was an easy decision to bring him here," the manager said.
"He is experienced, a very good professional and he will add competition in defence."
Fernandez made 118 appearances for Swansea, and has 32 Argentina caps to his name. He is eligible to face Tottenham in Saturday's season-opener.
9 August 2018