The 26-year-old North-Eastener, who was out of contract at Ipswich, is hungry to help Tony Mowbray's squad achieve promotion next season.
"I'm pretty excited," he says. "I wanted a fresh challenge and when the opportunity to join Middlesbrough came in, I just couldn't turn it down.
Former Ipswich manager Roy Keane paid £2.6m to bring Leadbitter from his old club Sunderland to Portman Road in summer 2009 and the player spent three seasons in East Anglia, making 116 league appearances and scoring 13 goals.
"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, 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."
Born in Chester-le-Street, Leadbitter joined hometown club Sunderland as a 16-year-old and progressed through the ranks to spend four seasons in the side as an attacking midfielder.