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Defoe signed a multi-year contract for an undisclosed fee.
The 25-year-old has been unhappy recently at White Hart Lane due to lack of regular football.
"He's a great goals-corer and he's a great signing for this club," Portsmouth manager Harry Redknapp told Sky Sports.
"He needs to play. He's too good a player not to be playing every week."
Defoe played for Charlton, West Ham, under Redknapp himself, before joining the Spurs.