"As much as I love the club I feel now is the time for me to move on," said Defoe.
"This is very much a career decision.
"I am very ambitious and hungry to achieve at the highest levels of the game for both club and country.
"The club staff and especially the fans are very dear to me and have been unbelievable in the early part of my career.
"It will always be a special place for me to play."