Luke Ayling has joined the club on loan until the end of the season.
The 32-year-old joins from Leeds United, where he made almost 300 appearances including almost a century in the Premier League.
The former Arsenal trainee was a member of Leeds United's promotion-winning side of 2019/20 and was also part of the Elland Road squad that reached the Championship play-offs the previous season.
The Lambeth-born Ayling totals 550 career appearances and had spells with Yeovil Town and Bristol City before joining Leeds in the summer of 2016.
The player, who becomes the second loan man from Elland Road, joining Sam Greenwood, is out of contract with Leeds in the summer.
Ayling is Boro's second signing of the January window - following Finn Azaz into the club - and head coach Michael Carrick was delighted to complete the deal.
"We're delighted to bring Luke to the club," he said. "He is a player with good experience, he's a promotion winner, and he's played at the highest level.
"He's a terrific character as well and we see him as a great fit for what we have here. He's exactly what we're looking for at the club."
The defender will be available for Saturday's trip to Millwall. He will wear the number 12 shirt.