Muto was born just outside Tokyo, before moving to Mainz in 2015, his first club outside of Japan.
In three seasons in Germany he has scored 20 Bundesliga goals in 66 appearances, in addition to making two appearances in the Europa League during Mainz's campaign two years ago.
He will sign for Newcastle subject to a medical and personal terms being agreed, becoming the fourth sigining of the summer for Rafa Benitez.
In a statement, Mainz said: "We have had had good and productive talks with Newcastle, and come to an agreement in principal."
The deal should pave the way for Aleksandar Mitrovic to move to Fulham, providing the London club can stump up the cash.
The move comes mere hours after Newcastle announced the signing of Swiss international, Fabian Schär, from Deportivo La Coruña.
26 July 2018