Taking to Instagram, 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."
The verdict
Grant Leadbitter has been such a massive player for North-East football.
His allegiance to Sunderland is something really special and as one of their own, it's been great to see him playing with the heart of a genuine Black Cats fan.
He also made a huge impact with Middlesbrough as their captain and helped them to secure promotion to the Premier League.
We don't know whether retirement is on the cards for the midfielder just yet, but if it is, then there's no doubt that Leadbitter will be regarded as one of the most universally popular players to come from the North East in recent years.