Seb Hines has moved to Major League Soccer side Orlando City on a six-month loan.
The 26-year-old defender, who has an American father, has an option to extend his stay in the United States if the move works out.
Hines featured for Boro in the first five games of this season before joining League One side side Coventry City for three months, making nine appearances for the Sky Blues.
He is the longest-serving player in Aitor Karanka's squad, having made his Boro debut as far back as January 2007 when he was on target in a 3-1 FA Cup replay victory over Hull City.
Hines, who has also enjoyed loan spells at Derby and Oldham, has made 69 appearances for Boro in an injury-blighted career.
He teams up with Orlando as they prepare for the start of their 2015 season.
"We're very pleased to have Seb join us," said Orlando Head Coach Adrian Heath. "His experience will be very valuable for our team's competitive position in our first MLS season. We look forward to start working with him."