Sol Campbell has been confirmed as Portsmouth's captain for the new season while former Arsenal team-mate Kanu has signed a one-year deal at Fratton Park.
Kanu, who was Portsmouth's top scorer last season, has the option of a further year depending on appearances.
Manager Harry Redknapp said: 'I am pleased he has decided to stay.
'He was our top scorer last year after signing on a free transfer and now he has trained with us again and looked excellent.'
Zambian striker Collins Mbesuma, however, has joined former England star Andy Cole in quitting Pompey without starting a match since he signed for the club's former boss Alain Perrin.
Mbesuma, a £500,000 signing from South Africa's Kaizer Chiefs in the summer of 2005, has gone to Turkish club Bursapor on a free transfer.