The 23-year-old, an unused substitute for Boro at West Brom last weekend, has joined the Championship promotion hopefuls for an undisclosed fee.
Reach, a product of the Boro Academy, made a goalscoring debut for the club in May 2011 when he scored in a 3-0 win against Doncaster Rovers and has since clocked up six goals from 70 appearances in all competitions.
An England international at both Under-19 and Under-20, Chester-le-Street-born Reach has had loan spells at Darlington, Shrewsbury Town, Bradford City, and Preston North End, where he made 36 appearances last season.
The left-sided player has not featured on the pitch for Boro this season and departs with best wishes of everyone connected with the club.
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