Jermain Defoe and Bradley Lowery
Bradley, from Blackhall Colliery, County Durham, was diagnosed with the disease when he was 18 months old. He underwent treatment and was in remission, but relapsed last year.
His plight touched the lives of many, and well-wishers raised more than £700,000 in 2016 to pay for him to be given antibody treatment in New York.
But medics then found his cancer had grown and his family was informed his illness was terminal. His death was confirmed on social media by his parents.
The posting read: "My brave boy has went with the angels today.
"He was our little superhero and put the biggest fight up but he was needed else where. There are no words to describe how heart broken we are."
Jermain Defoe has paid an emotional tribute to "best friend" Bradley Lowery after his death from a rare form of cancer.
The England striker struck up a close friendship with Bradley during his time at Sunderland. The pair first met when Bradley was mascot for Sunderland as his family started their campaign to raise awareness for the rare disease.
Bradley Lowery died on Friday, aged six, leading to tributes from all across the game.
Jermain Defoe said on Twitter: " Goodbye my friend, gonna miss you lots.I feel so blessed God brought you into my life and has some amazing moments with you and for that I'm so grateful. I'll never forget the way you looked at me when I met you for the first time, the genuine love in those cute eyes. Really finding it hard to find words to express what you mean to me.Your courage and bravery will continue to inspire me for the rest of my life. You will never know what a difference you made to me as a person. God has you in his arms and I will always carry you in my heart.
Sleep tight little one. My best friend."
Source Twitter @IAmJermainDefoe