The 26-year-old, who can play at centre-back, left-back or in a holding midfield role, becomes the Magpies' eighth signing of the summer and has penned a five-year deal at St James' Park.
Clark, capped 19 times by the Republic of Ireland, could make his debut for Newcastle in their opening Championship fixture against Fulham at Craven Cottage on Friday, live on Sky Sports 1 HD from 7.00pm.
"Newcastle is a massive club so I'm really happy to be here," Clark 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.
"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."
Clark came through the youth system at Villa and made his debut for the club in August 2009 in the 2-0 victory over Fulham.
He went on to make over 150 appearances for Villa and becomes the sixth player to leave the club this summer following their relegation from the Premier League.
Newcastle 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."