The 20-year-old midfielder has agreed a two-year extension that sees him committing himself to the club until July 2014.
Smallwood joins Marvin Emnes, Jason Steele, Joe Bennett, Seb Hines, Luke Williams, Cameron Park and Connor Ripley, who have all signed extended contracts recently.
It is further good news for manager Tony Mowbray following the decision earlier today of Julio Arca to sign a new two-year contract.
"I'm flattered and honoured to be able to extend my contract with Boro," said Smallwood. "All I have ever wanted to do is play for this club and I'm glad we have been able to sort all the details so quickly.
"It was a dream for me to break into the team for the first time last season. I was lucky enough to get quite a few games under my belt and I'm determined to build on that progress this year."
Source: Northern Echo
Source: Northern Echo