The 26-year-old was out of contract at Portman Road and has penned a three-year deal with Tony Mowbray's men.
"I'm pretty excited," Leadbitter told Boro's official website. "I wanted a fresh challenge and when the opportunity to join Middlesbrough came in, I just couldn't turn it down.
"I just want to play for Middlesbrough and get the club in the Premier League."
Mowbray is pleased with the signing, particularly in light of Barry Robson's departure to MLS outfit Vancouver Whitecaps.
He said: "In my mind, we are singing a player who has matured to have 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."