Tony Mowbray has bolstered his squad with the capture of the German defender Maximilian Haas on an 18-month contract.
The 25-year-old Bayern Munich II captain has been signed as a replacement for David Wheater, who moved to Bolton Wanderers earlier this month.
The former Eintracht Freising star featured in 62 matches for Bayern's second team, who play in the third tier of German football.
Mowbray said of his latest signing: "Maximilian was very keen to come here and test himself in English football. It's a challenge for him and his arrival will create additional competition for places in that area of the pitch.
"He's a real competitor and is comfortable with the ball.
"The challenge for him is to acclimatise to the English game and then take any opportunity that comes his way."
Meanwhile, Boro continue to be linked with Morocco international Merouane Zemmama.
It is anticipated that the 27-year-old will move to the Riverside Stadium on loan until the rest of the season before completing a permanent transfer in the summer.