Muto completed a medical and agreed personal terms last week, but his move was only completed yesterday after receiving a work permit.
The 26-year-old, who is Newcastle's first Japanese player, has signed a four-year deal with the Magpies.
"I am really looking forward to playing in front of the fans," Muto told the club's official website.
"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."