Rafa Benitez has described the decision to sign Federico Fernández from Swansea City as an "easy" one.
The defender joins on an initial two-year deal for an undisclosed fee, understood to be £6million.
Benitez knows the centre back from their time together at Napoli and he was pleased to get the deadline day deal across the line.
"I know Federico well from our time together at Napoli and it was an easy decision to bring him here," said Benitez.
"He is experienced, a very good professional and he will add competition in defence."
Speaking about his arrival on Tyneside, Fernández 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."
He has been handed the No.18 shirt and will be eligible to face Tottenham Hotspur on Saturday.
The 29-year-old Argentine has won 32 full caps for his country.
He began his career in his homeland with Estudiantes before moving to Napoli in 2011, he won the Copa Italia with Gli Azzurri in 2014 as well as playing in the Champions League.
Fernández signed for Swansea is the summer of 2014 and made more than 100 Premier League appearances for the Welsh club.
Swansea were understood to be holding out for £10million for the international, as they look to cut their costs after relegation to the Championship.
But the two clubs agreed a £6million fee for the centre-half earlier today and he had his medical this afternoon.