The terminally ill boy, whose battle with the rare childhood cancer neuroblastoma has touched people around the world, is spending his big day in hospital ahead of a big party later this week, which Defoe will attend.
The England forward posted a collage of pictures of himself with Bradley on Twitter on Wednesday morning and wrote: "Happy Birthday to my best mate @Bradleysfight
Hope you have a special day and I'll see you later this week..
Bradley's family said earlier this week that he was feeling "a little better", having been in great pain lately
On the Facebook page the family use to update the thousands of well-wishers, they said on Wednesday: " Bradley has had a settled night and he has had a lovely morning opening cards and presents
He is worn out now so he is having a nap recharging his batteries for later
"We have had lots of fun reading out his birthday wishes on his crowd funding and birthday page
Keep them coming and I'll get through as many as possible."
Bradley will have a tea party in hospital with family members on Wednesday, before a party in his home village of Blackhall, County Durham, on Friday.
Bradley has been a mascot for Sunderland and England several times and Defoe carried his young friend onto the pitch at the Stadium of Light for the last match of the season last weekend.
Premier League champions Chelsea invited Bradley to Stamford Bridge for the Black Cats' last game in the top flight on Sunday, but the family said he was "not well enough to go".
There were many other birthday wishes sent Bradley's way on Wednesday.
Sunderland posted: " Not only is it International Cup final day, but it's also a very special day for @Bradleysfight..
Many happy returns little man"
Newcastle, Everton and Chelsea were among other clubs to wish him well, along with official England team Twitter account
And world governing body FIFA posted: "To wish a very special young man a happy 6th birthday, head to his Facebook page #HappyBirthdayBrad"
Source : PA