The versatile 26-year-old joins from Aston Villa for an undisclosed fee and has put pen to paper on a five-year deal at St. James’ Park.
Clark came up through the ranks at Villa Park before making his debut in August 2009, while still a teenager.
It wasn’t long before he became a regular in the Villa side and over the following six seasons made a further 158 appearances, scoring ten goals.
The centre-back has also won 19 caps for the Republic of Ireland, representing his nation at Euro 2016 this summer.
“Newcastle is a massive club so I’m really happy to be here,” said Clark.
“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.
“Things seem to have progressed really quickly over the last few days but I’m delighted to have got it all done and be a part of this squad.”
Newcastle United manager Rafa Benitez added: “Ciaran is a very versatile player, as a left-footed centre-back he can also play left-back and also as a defensive midfielder.
“He has a lot of experience of English football and will add good balance and competition to our squad.
“I am sure he will progress with us here at Newcastle, we are very happy for him to join us.”