Defoe left a pre-season training camp in Spain with his new club Bournemouth so he can attend the service in Blackhall, County Durham.
Bradley died a week ago in his parents' arms at his home in Blackhall.
He had a close friendship with Defoe, who said Bradley had been his "best friend" and would continue to inspire him.
Bournemouth posted a message on the club's official Twitter feed on Thursday which said: "@IAmJermainDefoe has today departed our pre-season training camp and returned to the UK for the funeral of Bradley Lowery."
Defoe, 34, signed a three-year deal with the the club after leaving Sunderland following their relegation from the Premier League.
Bournemouth are currently away in Marbella, where they will take on Portuguese side Estoril in a friendly on Saturday.
Bradley was a mascot for Sunderland several times last season and was also a mascot for England at Wembley.
The funeral will take place at St Joseph Church at 11.15am, and friends and family have been encouraged to wear football shirts to help celebrate his life.
Speakers outside will allow anyone who is unable to get into the church to listen to the service.
A statement from the twitter account @Bradleysfight said the funeral was "open to everyone who would like to come and celebrate Bradley's life and pay their respects to show him how much he was loved".
It added: "You can wear whatever you want for the funeral but the family and friends have chosen the theme cancer has no colours. So they are wearing football tops, it doesn't matter what team or colour."
Supporters had hoped a massive fundraising effort last year could get Bradley to the US for pioneering treatment for the childhood cancer neuroblastoma, but his heartbroken family announced at Christmas that his cancer was terminal.