Kanu, who was released by West Bromwich Albion after they were relegated to the Championship, signed a one-year contract with Pompey.
The 31-year-old is likely to start Portsmouth's Premiership opener against Blackburn Rovers on Saturday as striking duo Tresor Lua Lua and Benjani Mwaruwari are injured.
Kanu joined Arsenal from Inter Milan in 1999 where he scored 3o goals in 63 appearances and won two Premiership titles with Gunners.
The former Ajax Amsterdam striker moved to West Bromwich in the summer of 2004 on a free transfer but he failed to impress, having scored just nine goals in two seasons.
Kanu is Portsmouth's fifth signing in the off-season following the arrival of Glen Johnson, Sol Campbell, David Thompson and goalkeeper David James.