Having made 218 appearances for Toulouse, his only club, Sissoko has signed a six-and-a-half-year deal with Alan Pardew's side.
A six-cap France international, he said: "I am delighted to be a Newcastle United player.
"This is a massive club playing in the best league in the world, and I believe my style of play is suited to the Premier League.
"I have heard lots about how fantastic the Newcastle supporters are and cannot wait to get started.
"I am rising to a new challenge by coming here and want to firstly help the team improve this season and then progress after that."
Toon boss Pardew said: "He will bring drive and energy to the squad, he is a winner and that's why I'm especially pleased to have him with us.
"What all our signings this window bring is essential competition for places and that is great for the remainder of the season."