Bailey follows striker Kris Boyd to the Championship side as manager Gordon Strachan looks to push his team up to the Premier League.
Manager Gordon Strachan told Boro’s official website: “We’re delighted to sign Nicky. He scores goals and all different kinds of goals.
“He can play left side of midfield or centre midfield and he’s self-motivated. There’s also a devil inside him when he plays and you sometimes need that.”