Jacob Butterfield has joined Huddersfield in a swap deal which sees Adam Clayton move to Middlesbrough.
Butterfield made 32 appearances for Boro last season but did not feature in their opening Championship game as he prepared to join the Terriers.
Moving the other way on a four-year deal is former Leeds midfielder Clayton, 25, who has spent the last two years with Huddersfield since leaving Elland Road.
Boro head coach Aitor Karanka told the club's official website: "Adam is another player who will improve our squad and I'm delighted to welcome him to Boro.
"He is a strong player and a strong character and just the sort of person I want in my squad."