Riviere moves to St James' Park for an undisclosed fee, boosting a forward line that had looked short of depth following the end of Loic Remy's loan spell at the club.
The 24-year-old is the latest recruit in what has been a busy transfer window for Newcastle, who have already signed Ayoze Perez, Jack Colback, Siem de Jong, and Remy Cabella, while Daryl Janmaat is poised to join in the close-season.
Former Saint-Etienne and Toulouse striker Riviere scored 13 goals in 36 appearances as Monaco finished second in Ligue 1 term.
And he will hope to build on his decent form with the principality club as he attempts to make a success of his career in the Premier League.
Riviere told Newcastle's official website: "I'm very happy to be here at Newcastle.
"When the club spoke to me I said yes immediately. It was an easy decision because Newcastle is a big club with great fans and a good stadium.
"My dream was always to play in the Premier League and Newcastle have given me the opportunity to realise that."
Newcastle manager Alan Pardew added: "This is an important signing because the striking area is one we really had to improve.
"We have looked at Emmanuel for a while and we know he will score goals for us.
"He is quick, hungry for success and will love having 50,000 Geordies cheering him on and singing his name.
"I am really looking forward to integrating Emmanuel in the group and cannot wait to see him in action for us."
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