Spurs chairman Daniel Levy and Pompey chief executive Peter Storrie have been in talks over the weekend to negotiate a deal in the region of £15million.
The 26-year-old has agreed terms - thought to be a five-year contract - with the club he left just under 12 months ago.
A statement from Spurs said: “Personal terms have been agreed and the striker will undergo a medical."
Pompey later released a statement, saying: "Portsmouth can confirm a fee has been agreed with Tottenham for Jermain Defoe."
Spurs boss Harry Redknapp bought Defoe for Pompey in the 2008 January window and his return had been suggested since the change of manager at White Hart Lane two months ago.
Tottenham fans continued to sing his name while Defoe went on a run of eight goals in his first seven Pompey games. He has added nine goals in this campaign.
“I wouldn’t have sold him if I was manager at Portsmouthand Tottenham came back in,” Redknapp admitted. “I’d be keeping him.”
Redknapp has prioritised a striker and a left-sided midfielder in the current window, with Middlesbrough’s Stewart Downing also subject of a bid from Spurs.
Ghana midfielder Stephen Appiah is on trial at Spurs this week and could sign until the end of the season.
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