The club has completed the signing of Sierra Leone international Alex Bangura.
The 24-year-old becomes the club’s 12th signing of the summer after penning a four-year contract.
Alex joins from Dutch Eerste Divisie side SC Cambuur, where he has made over 100 appearances.
A left-sided player, who can play at the back or on the wing, Alex started his career in the youth set-up at Feyenoord before joining Cambuur.
He brings plenty of experience, having played in the Dutch top division, and he has also won five caps for his country.
Boro head coach Michael Carrick was delighted to welcome Alex to the club.
"He's a really exciting signing," he said. "He's a left-back and he's captained his team.
"He's a good character with a great personality and he won't have any problem fitting into the group."