Newcastle have completed the signing of Federico Fernandez from Swansea.
The fee is understood to be in the region of £6m and the player has signed an initial two-year contract.
The Argentine international had a medical on Tyneside on Thursday afternoon after a deal was struck between the clubs at lunchtime.
Newcastle turned to Fernandez when their move for Barcelona's Marlon Santos fell through after the player failed to gain a work permit.
Speaking about his move, Fernandez said: "It was a long day but I'm really happy to be here. It's a new challenge in my life and I'm very excited to meet all the players and staff here."
Fernandez, who has made 32 appearances for Argentina, signed for Swansea in the summer of 2014 and made more than 100 Premier League appearances for the Welsh club.
Earlier in the window, Newcastle brought in Swiss defender Fabian Schar from Deportivo La Coruna, but Magpies boss Rafael Benitez was keen to sign Fernandez to compete with Schar, Jamaal Lascelles and Ciaran Clark.
French defender Florian Lejeune recently had surgery to repair his anterior cruciate ligament after injuring his knee during pre-season, meaning he will miss much of the upcoming campaign.
Benitez explained: "I know Federico well from our time together at Napoli and it was an easy decision to bring him here.
"He is experienced, a very good professional and he will add competition in defence."
Fernandez will wear the number 18 shirt for Newcastle United and will be eligible to face Tottenham Hotspur on Saturday, live on Sky Sports Main Event.