The deal for the 21-year-old is believed to be worth about £11m.
"I am very happy he will come to us and hopefully he will have a good time with us," said manager Ralph Hasenhuttl after his side beat West Brom 2-0 on Sunday.
"He's a very smart one, a very clever one, he reads the game very well. He's also quick and dynamic."
Hasenhuttl added: "He is an important signing for us, because in midfield we only have so many alternatives, and he showed in France that he is a fantastic midfield player. He is a player who I think fits perfectly in our team."