Miller joined Forest last summer but only managed four goals in 23 appearances last season.
The 25-year-old, who was left out of Forest's opening games of this season, jumped at the chance to rejoin Middlesbrough manager Tony Mowbray having previously worked with him at West Bromwich Albion.
"I'm very happy and excited to be reunited with Tony Mowbray and [assistant boss] Mark Venus," said Miller.
"Tony gave me my first shot of playing regular first-team football and I will always be grateful to him for that. Playing for Tony was the best time of my career so far. I'm just relishing the opportunity to play for Boro. I was excited when I heard Tony Mowbray was interested in me.
"I had a few different options but Tony knows what I can do and I really enjoy working with him."
Middlesbrough's next match comes against Crystal Palace tomorrow.