On 26 March 2015, Tiendalli was loaned out to promotion-chasing Middlesbrough of the Championship for the remainder of the 2014–15 season. He made his Boro debut away to Watford on 6 April 2015, starting in a 2–0 loss. He sat out the next four matches for the club and did not make an appearance until the last game of the season, a 0–0 draw with Brighton & Hove Albion. These were his only two appearances for Middlesbrough. Middlesbrough elected to terminate his loan deal, allowing him to return to his parent club.