A former Manchester United trainee, Fletcher scored eight goals in 27 appearances during a six-month loan spell with Barnsley last year.
Fletcher is Boro manager Garry Monk's sixth major signing of the close season transfer window after the arrival of Britt Assombalonga, Martin Braithwaite, Cyrus Christie, Jonny Howson and Darren Randolph at the Riverside Stadium.
(Reporting by Hardik Vyas in Bengaluru, editing by Ed Osmond)
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