In six and a half seasons on Teesside, Leadbitter clocked up almost 250 appearances in all competitions and 32 goals, including 12 in a productive 2014/15 campaign as Boro reached the Championship Play-Off Final.
The following year, Leadbitter lead Boro to automatic promotion to the Premier League under the management of Aitor Karanka.
But he has found first-team football hard to come by this season with only two league appearances, and the writing was on the wall following the arrival of fellow midfielder John Obi Mikel.
It is thought Leadbitter was keen to stay at the Riverside - but he could not refuse the opportunity to return to the club where he started his career.
“I’m absolutely delighted to be back, I’m buzzing,” he told the club’s website.
“When you’re a born-and-bred Sunderland fan and there’s an opportunity to come back to the football club, well, I was never going to say no, I can tell you that.”
Leadbitter’s departure is the second of the day for Boro, after defender Danny Batth was recalled from his loan from Wolves and sold to Stoke City.