Winger Mustapha Carayol has left the Riverside Stadium after his contract was terminated by mutual constent.
Carayol, 27, will now join up with Nottingham Forest on a two-year deal.
The Gambian international made over 50 appearances for Boro after joining from Bristol Rovers in 2012, netting 12 times.
Having previously spent time on loan at Brighton, Leeds and Huddersfield, Carayol has now departed on a permanent basis.
Elsewhere, Aitor Karanka expects it will be a quiet deadline day for the Teessiders after they completed most of their business early in the summer.
Speaking to the club’s official website, Karanka hinted that he is happy with the squad he has.
“I’m relaxed,”he said.
“We have been prepared and the club has worked very hard over the summer to do what we wanted to do, and for this reason I am relaxed and happy.”