The 23-year-old, who joins the Magpies for an undisclosed fee, has put pen to paper on a six-and-a-half year deal at St James' Park.
He will wear the number seven shirt and could make his debut in Tuesday night's visit to Aston Villa.
"I am delighted to be a Newcastle United player," Sissoko told the club's official website. "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 made 218 first-team appearances for Toulouse and he has been capped five times at senior international level.