The club have completed the signing of Danish defender Lukas Engel, subject to international clearance and receipt of visa.
The 24-year-old left-back joins from Danish Superliga side Silkeborg and has signed a four-year contract.
The former Under-21 international has already made almost 200 career appearances including a large number in the Danish top-flight.
Engel also featured for Silkeborg in the Europa Conference league.
Boro head coach Michael Carrick said: "Lukas is a good age, he's already gained experience to a point and looking to make the next step in his career.
"He's desperate to do well and you can tell he’s very happy to be here. I'm looking forward to working with him."
He becomes the club’s eighth senior signing of the summer.