The 35-year-old was one of seven players released by the Black Cats earlier this week following their play-off semi-final defeat to Lincoln City, bringing to an end the midfielder’s second stay at the club.
Having initially progressed through the club’s academy, Leadbitter made over 120 senior appearances on Wearside before taking in spells at Ipswich Town and Middlesbrough.
He returned to the Stadium of Light in 2019 and appeared a further 80 times for the club, while delivering some memorable moments along the way.
And Leadbitter has now delivered an emotional message of thanks to the club’s fanbase – while also apologising for being unable to guide the club out of the third tier.
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Many supporters have called for the boyhood Sunderland fan to be handed a coaching or ambassadorial role at the club moving forward, but the man himself says he will now take his time to evaluate his next move.
Posting on his Instagram page, Leadbitter said: "Sunderland fans, first of all, an apology. I'd have loved to have helped deliver Premier League football back to Wearside during my second spell, never mind promotion to the Championship.
"But thank you, you’ll always be in my heart.
"It would have been perfect to have finished my time back at the Stadium of Light with promotion but it wasn’t to be.
"There are disappointments, of course there are, but there are great memories too throughout my time playing for Sunderland over both spells.
"I have learned so much during my second spell at the club, some good and some bad, and I will always learn from such experiences.
"I will remember captaining the team one last time, with fans back at the stadium for the Playoff 2nd leg, and the incredible noise you created last Saturday.
"We fell short in succeeding.
"Hopefully that can be achieved next year, the loudest cheers coming from me, celebrating as a fan.
"It’s almost two and a half years since I rejoined Sunderland, where it all started as a 6-year-old, and I will always be immensely proud to have worn the red and white shirt in every game I played.
"It has meant the world to me and my family.
"I will always remember the few people at the club who helped me through some tough times personally last year, that will stay with me.
"I am now going to enjoy a break with my family, consider my options and decide my next step. I haven’t decided yet.
"Sunderland was the club I supported as a schoolboy, where my career started and I will always love the club.
"Please support the young players coming through next season and help them develop.
"The good times will come back.”
Also leaving the club this week have been Max Power, Chris Maguire, Callum McFadzean, Josh Scowen, Remi Matthews and Conor McLaughlin.
Former captain Power – who has been linked with Ipswich Town – sent a message to supporters on Thursday.