The Bournemouth striker struck up a close friendship with the youngster during his time at the North East club and went to his sixth birthday party and his funeral in July.
Bradley died from the rare childhood cancer neuroblastoma, but his positive approach to life with the disease, during which time he joined Defoe in leading out England at Wembley, gripped the nation.
Defoe posted a heartfelt message dedicated to his friend on Thursday, accompanied by a montage of photos of the pair spending time together.
“Bradley would have been seven years old today,” he wrote.
“I know you’re up there smiling down on us, especially your mum, dad and brother, and I bet you’re throwing another memorable party.
“Happy birthday, mate. We miss you and love you more than you’ll ever know…”
The funeral service for Bradley took place at St Joseph’s Catholic Church in County Durham, which Defoe attended alongside his then teammates John O’Shea, Lee Cattermole and Vito Mannone.
Former Sunderland boss David Moyes also attended.
A piper led the coffin, which was in a carriage pulled by white horses draped with Sunderland’s red and white stripes, while friends and relatives walked behind dressed in superhero costumes and football shirts with “Bradley” and the number six printed on the back.
Sunderland have since started a campaign to fund a holiday home for sick children and their families, dubbed the “For Bradley” initiative.