The Cherries confirmed on Thursday afternoon that the 34-year-old had joined the club on a free transfer after a clause in his contract allowing him to do so had been triggered by the Black Cats’ relegation from the Premier League.
Defoe leaves the Stadium of Light with a heavy heart after building up a close friendship with inspirational cancer fighter Bradley, 6.
The youngster led his beloved Black Cats out against Everton at Goodison Park in March, just days after it had emerged he had a new tumour.
And Defoe admits that day on Merseyside was one of the highlights of his time with the Wearsiders.
He wrote in an open letter to fans: “I’d like to thank all the Sunderland fans for their support and everyone else involved with the football club.
“I had an amazing time with many great memories.
“I’m going to miss so many people at the club from the canteen staff and the chef to the laundry girls and the cleaners who always made the boys smile no matter what.
“I feel blessed to have played for such a great football club with some of the best fans I’ve ever come across.
“The highlight of my time has to be walking out with Bradley against Everton and that volley against Newcastle – a goal people still come up to me and talk about!!
“I want to wish everyone at Sunderland the best for the future.
“There’s some talented players there and some great youngsters pushing through, so I can’t wait to see you back in the Premier League.
“I’m looking forward to my new challenge and can’t wait to get started at AFC Bournemouth!”