Matt Crooks has become Boro’s fifth signing of the summer, joining from Rotherham United.
The 27-year-old has signed a three-year deal after completing the formalities of his move at Boro’s training camp in the south west.
Boss Neil Warnock said: "I'm delighted to have got him, I've always liked Matthew, for many years.
"He can play in different positions, and he's a wholehearted player that I think the fans will take to.
"I'm sure he'll have a good few years at Middlesbrough."
Crooks clocked up almost 100 appearances for the Millers after joining in 2019 and was a promotion winner with the club. Last term, he scored seven goals in 42 appearances and was named Championship Player of the Month in January.
The Leeds-born midfielder is a product of the Manchester United Academy, and his previous clubs include Huddersfield Town, Accrington Stanley, Rangers, and Northampton Town. He has made almost 250 career appearances, scoring 43 goals.
Crooks joins Joe Lumley, Sammy Ameobi, Uche Ikpeazu and Lee Peltier in joining the club this summer as Warnock looks bolster his squad for the new season.
Crooks is likely to make his first appearance in a Boro shirt at Plymouth on Friday.