Defoe has reunited with boss Harry Redknapp who he played under at Portsmouth, Bournemouth and West Ham and was delighted with the reception he received from his old fans; Defoe also having played at Tottenham.
"It's great to be back," the England striker told Sky Sports.
"I'm looking forward to the challenge.
"It's hard to put it into words, it's brilliant. I've been looking forward to it all day.
"A number of things [made me return]. Obviously everyone knows the manager was a major factor.
"My family still live in London and the fans were a factor too. You can tell by the reception I got."