The 29-year-old becomes Garry Monk’s first summer signing as Boro boss, as Boro look to bounce straight back up to the Premier League at the first attempt.
The fee is undiscolosed, however is thought to be around the £5 million mark.
On Howson’s departure, Norwich City Sporting Director Stuart Webber said “Jonny made it very clear during my first week in the job he didn’t want to be here and wanted to move back to the North.”
“We spoke about a new contract but he was insistent. Jonny is a good guy who was great for this club but sometimes you have to accept it becomes about getting the best deal for the football club.”
Leeds United had been interested in re-signing the midfielder, however Howson opted for a switch to Boro.
Howson made 188 appearances for Norwich City and was named their player of the year in 2016. He has captained both Leeds and Norwich, and scored over 50 goals.