The club have signed winger Nathaniel Mendez-Laing on a contract until the end of the season.
The 28-year-old is well-known to Boro boss Neil Warnock after the pairing won promotion to the Premier League at Cardiff City in 2017/18 when Mendez-Laing netted seven goals in 42 appearances for the Bluebirds.
The Birmingham-born flyer brings a wealth of experience, having made over 300 career appearances since breaking through the ranks at Wolverhampton Wanderers over a decade ago.
The delighted Boro boss said: “I know Mendez well and he’ll give us an added dimension. It’ll be couple of weeks before he’s match fit but it’ll be great to have him for this run-in now.
“We have been short of wide players and he is a player with something to prove. We’re looking forward to having him here.”
Mendez-Laing is the fourth signing of the January transfer window with Jordan Archer, Darnell Fisher, and Yannick Bolasie already arriving at the club.