The 23-year-old midfielder is the club's fourth signing of the week and the latest Frenchman to move to St James' Park.
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."
Sissoko's signing comes at the end of a day during which Pardew was boosted by wantaway captain Fabricio Coloccini's decision to remain with the club until the end of the season at least.
"This transfer window has proved very productive for us and Moussa arriving today is a massive plus for the club," 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."
Sissoko joins compatriots Yoan Gouffran, Massadio Haidara and Mapou Yanga-Mbiwa in joining this week, while Mathieu Debuchy signed on January 4.