The 31-year-old, who first joined Tottenham from West Ham in 2004 and returned in 2009 after a short spell at Portsmouth, will continue to be available for the London club until February 28.
"Everybody knows how much this club means to me and I will play my heart out for the team and the fans until my last kick for the club," Defoe told Tottenham's website (www.tottenhamhotspur.com).
"I have had a fantastic time here and the fans have always been amazing to me."
Defoe is a regular member of Roy Hodgson's England squad but a move to north America, where the season starts in March, could impact on his chances of being on the plane to Brazil for this year's World Cup finals.
However, Defoe, who has scored 19 goals for England in 55 appearances, says it is an opportunity he could not refuse.
"I am grateful for the opportunity for myself and my family to experience living overseas and for me to play in the MLS with a contract that will take me to the age of 35," he said.
Defoe, who in November became Tottenham's record scorer in European competition with 23 goals, has scored 142 times for Tottenham although in recent seasons he has often been used as a substitute.
He is fifth on Tottenham's all-time scoring chart.
(Reporting by Martyn Herman)
