The 26-year-old, who joined the Tractor Boys in a £2.6million deal from Sunderland in summer 2009, made 116 league appearances for the club, scoring 13 goals.
Leadbitter has penned a three-year contract at the Riverside Stadium.
"I'm pretty excited," he 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."