The Japan striker has been granted a UK work permit after Mainz accepted an offer from the Toon last week.
Muto said, “I am very happy to be a player for Newcastle United. I am really looking forward to playing in front of the fans.
“As the first Japanese Newcastle United player, I am honoured to be a player for such a big club. I hope to achieve good results – that is what I am here to do. I would like to perform well and leave my name in Newcastle's history."
Manager Rafa Benítez added: “I am really pleased to have Yoshinori Muto with us. We were following him for quite a while.
“Hopefully he can bring us his energy and work-rate and help the team to improve and be better for the new season."