Spanish-born Nsue, 24, has been capped by Equatorial Guinea and is expected to complete a three-year deal.
"He is a versatile player who I have known for five years. He is a good lad with a good mentality and approach to the game," Boro boss Aitor Karanka told the club website.
"I know the type of person he is, and that is important."
He added: "As a player he can play in different positions and I am delighted we have agreed a deal to sign him."