The defender has joined from Championship rivals Aston Villa.
The defender officially joined the Magpies on Wednesday as he completed a move from Championship rivals Aston Villa.
He becomes their seventh signing in what has been a busy transfer window for manager Rafa Benitez.
Those moves have come as they look to embark on a bright future under the Spaniard's leadership, starting with a return to the Premier League at the end of this season.
And he says that sense of the club moving in the right direction was a big factor behind the move.
"Newcastle is a massive club so I'm really happy to be here," he told the club’s official website.
"Importantly, it is a club that is moving in the right direction and pushing for the right things, so I'm really excited to now be a part of that.
It now remains to be seen what role Clark has to play in the Magpies' line-up next season.
The centre-back position has long been an issue for them but they have moved to fix that with the signings of Clark and Grant Hanley this summer.
With Chancel Mbemba and Jamaal Lascelles already at the club, there is now a fight on for first-team places and it will be interesting to see how often Clark features.