The 20-year-old stands at 6ft 4in, is athletic and can play in a number of positions.
Part of Middlesbrough's academy and development squad for more than a decade, he has occasionally featured in Boro's senior squad since early 2021.
He made several appearances at U18 and U23 level for Boro, netting goals against Blackburn Rovers and Burnley, as well as a hat-trick against Sunderland in 2020.
Fletcher made his senior debut for Boro in the Carabao Cup first round against Barnsley earlier this month.
He also made 15 appearances on loan for Hartlepool in League Two last season, scoring the winner in their 2-1 away win at Colchester in February.