Defoe was a regular visitor to Bradley during his time at Sunderland and often brought him out onto the pitch with him before matches.
Bradley passed away on Friday following a brave battle against the disease, and Defoe has now said goodbye to his "best friend".
Sleep tight little one... 💙 pic.twitter.com/iGqLXdvlVi— Jermain Defoe (@IAmJermainDefoe) July 8, 2017
The statement reads: "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."
Defoe, who has since left Sunderland following their relegation from the Premier League, broke down when asked about Bradley at a press conference with new club Bournemouth earlier this week.