During his time at Sunderland, the 34-year-old striker nurtured a close friendship with Bradley, who tragically died on July 7 from cancer aged just six years old.
On Thursday, the official Bournemouth Twitter account wrote: "@IAmJermainDefoe has today departed our pre-season training camp and returned to the UK for the funeral of Bradley Lowery."
The funeral will take place on Friday at St Joseph's Church in Blackhall Colliery, County Durham, with the memorial service also set to be played on loudspeakers outside. Bradley's family will then hold a private ceremony at a crematorium.
The Cherries are currently in Marbella, where they will take on Portuguese side Estoril in a friendly on Saturday.