It comes following his medical which was completed late on Monday night, and now the attacker has signed a long-term deal with the club.
Speaking to Newcastle's club website, the Belgian said: “I’m really, really happy to be here and very proud to be at such a big club.
“I spoke to the head coach and he really showed how much he wanted me to be here. In terms of my own career progression, it made complete sense to come so I didn't hesitate at all.
“I know the club, I know about the stadium, I know it's a fantastic atmosphere. I've followed the team. What I've seen is a team that's very ambitious, wants to attack, get forward, really chase teams and go after them, and that's really something I want to take part in as well.
“I'm still young, I'm still hungry, and that's why I'm here, and I want to make that progress with Newcastle. I can't wait to start the wonderful adventure here."
The highly rated attacker becomes Newcastle's eighth signing of the summer transfer window.
Newcastle boss Matthias Jaissle added: “I am very happy to welcome Matias to our club. We were all delighted when we knew we could complete this deal.
"Matias is a highly technical player with explosiveness in his game. He can score and assist, he has courage, and he has the ability to take players on. We’re sure our supporters will enjoy watching him play."