The former England international signed a six-month deal at the Riverside back in January.
However, despite insisting that the 34-year-old has been "fantastic" to have around, Boro manager Tony Mowbray has decided to part with the midfielder.
"He's been a delight to work with again, not just on the field. He scored two goals and I thought in his last home game he was the catalyst for us," Mowbray told the club's official website.
"Off the pitch he's been fantastic, entertaining the players, bringing quizzes on the coach for a five-hour trip and being a real good senior professional footballer for us. It was great to work with him again but I don't think we'll be continuing that into next season."
Dyer began his career with Ipswich Town, before turning out for Newcastle United, West Ham United and Queens Park Rangers prior to his stint on Teeside.