The 22-year-old has signed a five-year contract with the Magpies.
Thauvin scored 15 goals in his 81 appearances for Marseille.
“I am delighted to be a Newcastle United player,” said Thauvin.
“I have come from France to a really big club and am very happy to be here.
“It is a big honour to know that Newcastle have been after me for such a long time and that they wanted me that much.
“They have put their trust in me and I am looking forward to playing for the team.”
Newcastle United head coach Steve McClaren added: “Florian is a player the Club have been watching for a long time and I am delighted that we have been able to sign him.
“He is a perfect signing for this Club – someone who is young, with great potential and is one of the best young players in Europe.
“He is an exciting, creative talent who can score goals and make assists, and we believe he will have a very bright future at Newcastle.”