The 21-year-old has made 59 appearances for us, playing at right back and in the centre of defence.
Arriving from Southampton in July 2014, he made his competitive debut in our Community Shield victory over Manchester City. He has also been a regular for England Under-21s and made his full international debut against Norway in September 2014.
Calum heads to Middlesbrough for the opportunity to gain further first-team experience in the Premier League.
Everyone at the club wishes Calum well for his time with Middlesbrough.
The deal is subject to the completion of regulatory processes.