Nigeria international Kanu has agreed a one-month contract with Portsmouth, the Premier League side have announced.
The 33-year-old's contract at Fratton Park expired this summer but his new deal provides only temporary relief for the troubled club.
Portsmouth are in financial turmoil, with the attempted takeover by Dubai property developer Sulaiman al-Fahim still to be completed despite being agreed three months ago.
Manager Paul Hart has been unable to bring in anyone on a permanent basis, but the deal for former Ajax, Inter Milan, Arsenal and West Brom striker Kanu, who has scored 21 goals in 94 appearances for Pompey, will add to his options up front.
Pompey have raised funds by selling England internationals Glen Johnson to Liverpool for £17m and Peter Crouch to Tottenham for £9m this summer and meet Fulham on the opening day of the Premier League season.