Lewis O’Brien has joined the club on a season-long loan from Nottingham Forest.
The 24-year-old becomes the 10th arrival of what has been a busy summer for the club.
The highly-rated midfielder, who completed his medical and the formalities of his move on Wednesday evening, has made over 200 career appearances and brings a wealth of experience to Michael Carrick’s squad.
He clocked up over a century of appearances at Huddersfield and won the Terriers' POTY award before being snapped up by Nottingham Forest last summer on their return to the Premier League.
“Lewis has tasted success before and we’re delighted to have him on board,” said the Boro head coach.
“He’s got good experience, he’s a good age, and he offers something different to what we have here.
“His arrival gives us greater options, and we’re looking forward to working with him.”
Lewis has the added experience of playing in the MLS earlier this year after he was loaned to DC United.