The 31-year-old Republic of Ireland international has made 103 appearances for Toon since joining from Aston Villa in 2016.
After signing the new deal, Clark said: "It's a brilliant feeling and I'm delighted that I've managed to get it signed.
"My contract was ending in the summer; for me, I was trying to stay more focused on the games, getting back involved this season and trying to play games and I've managed to do that over the last few weeks.
"To be back playing was the real aim for me, then the contract has followed, so I'm delighted to concentrate now on the next couple of years."
Manager Steve Bruce also expressed his joy over retaining one of his key performers.
"I'm delighted Ciaran has signed a new deal. First and foremost, he is a very talented defender and he gives us great balance as a naturally left-footed centre-half.
"He has also become a very consistent performer and I'm pleased to see his hard work and commitment rewarded."
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