The former Sunderland midfielder's contract with the Tractor Boys was due to expire next month and he has sealed a move North to become the club's first signing of the summer
The transfer sees the Tractor Boys lose money on the player following his £2.6 million signing from Sunderland in 2009 but, with his contract expiring this summer, he moves to the Riverside Stadium for free.
Twenty-six-year-old Leadbitter has signed a three-year deal that begins in July and, following three seasons at Portman Road, is eagerly anticipating a new start.
Speaking to the club’s official website, he said: "I'm pretty excited. I wanted a fresh challenge and when the opportunity to join Middlesbrough came in, I just couldn't turn it down.
"I wanted a fresh challenge and when the opportunity to join Middlesbrough came, I just couldn't turn it down"
"I just want to play well for Middlesbrough and get the club in the Premier League."
Leadbitter's new manager, Tony Mowbray, is also excited about the purchase, insisting that he will be a good replacement for Barry Robson.
He said: "I am delighted we have managed to sign a player of Grant's quality, tenacity and competitive edge.
"In my mind, we are signing a player who has matured to have a good understanding of the game. Following the loss of Barry Robson [to Vancouver Whitecaps], he will ensure that we will retain the competitive experience that we need in midfield.
"We knew that Grant was out of contract and when a player of his qualities is in that position we obviously became interested. Hopefully he will be a core player for us in a side that will be pushing for promotion next season."