Redknapp originally lured Defoe to Pompey in 2008, for a deal worth up to £12m, when he was in charge at Fratton Park.
Defoe was paraded before the Spurs fans ahead of their home 4-1 Carling Cup first-leg semi-final win over Burnley, even though the Premier League had yet to approve the deal.
Rule L19 forbids players returning to clubs within a year, but the League can waive this at its discretion.
BBC Radio 5 Live's senior football reporter Ian Dennis explained: "From my understanding a sell-on clause worth about £4m by Portsmouth has been waived.
"While the package is £15m the initial outlay isn't even double figures in terms of millions. Any transfer upwards of £10m, Spurs must announce to the Stock Exchange and there has been no listing of this transfer."
Defoe scored 17 goals in 36 games for Portsmouth, comprising eight in his first seven matches and nine more this season.
Any deal for the forward, who has scored six goals in 32 appearances for England, also took into account monies owed to Tottenham by Portsmouth for Younes Kaboul and Pedro Mendes.
Spurs chairman Daniel Levy said: "We are delighted to welcome Jermain back to the club.
"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.
"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."
The frontman had also been linked with Manchester City and Aston Villa.
Former Spurs defender Gary Stevens is tipping the forward to be a big hit on his return to the club.
"Defoe is a proven goal-scorer with a terrific strike record and if you play him on a regular basis and give him the ammunition, he will get you goals. It is what he was born to do," he told BBC Sport.
"I'm sure when he moved to Portsmouth last January he didn't for one moment envisage going back to Spurs within a year.
Harry will know exactly what he's getting from Defoe - and how to get the best out of him
"But I believe he never wanted to leave the club in the first place really and it appears that he always had a great affection for Tottenham. It's a win-win for the club and the player."
It will be the third time Defoe has played under Redknapp as the duo were also together at West Ham near the turn of the century.
Stevens said: "Relationships do develop in football and Harry will know exactly what he's getting from Defoe - and how to get the best out of him.
"First things first, Jermain needs to get the goals that will help Spurs pull away from the drop zone.
"But Spurs are still in three cup competitions and they say football is all about timing - maybe Jermain's arrival is just in time for him to make a cup final appearance."
Redknapp is also keen to sign a left-sided midfielder in the January transfer window but Middlesbrough have already rejected two bids from the Londoners for Stewart Downing.
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