Defoe agreed terms to rejoin the club he left just over a year ago earlier in the week and was paraded to fans ahead of the Carling Cup semi-final first leg victory over Burnley - but the paperwork was not filed immediately.
The Premier League now have the 26-year-old's registration and he is free to play at the JJB Stadium this weekend.
"We are delighted to welcome Jermain back to the club," said Spurs chairman Daniel Levy.
Defoe revealed Spurs supporters have been asking him to return ever since he made the move to Fratton Park.
"It feels crazy," he said. "Since I left I've been getting texts and everywhere I go I bump into Spurs fans - it's just great to be back.
"The fans were probably the main factor. I spoke to my cousin and he said 'the fans love you there'. For me it is a strange feeling.
"When you get that many fans who appreciate what you do, it's hard to find words how I feel about it.
"I want to start afresh, like it's the first time, and hopefully score a lot of goals."
Levy only let Defoe go to Pompey as his contract was running down and he needed regular football.
"We had previously been keen for Jermain to sign a new contract to extend his initial stay but at that point in time he felt it was in his interests to move on in order to guarantee himself more starts," Levy added.
"I know the club is close to his heart and that he is thrilled to be back and to once again link up with Harry."
There are Premier League guidelines preventing players returning to a club within a year but that rule is to prevent abuse of the loan system - and nothing has been deemed sinister over Defoe's move.
Boss Harry Redknapp has hinted at using Defoe in attack with Roman Pavyluchenko.
"This is the third club I have signed him for because I know what a top-class player he is as well as a proven goalscorer," Redknapp said.
"I am pleased to have him here and I am sure the supporters are looking forward to seeing him back in a Spurs shirt."