Kanu signed a one-month deal with Pompey in August as talks over a longer agreement went on.
Diop’s new deal will keep him at Fratton Park until 2012.
“It’s good news. Papa is a strong midfielder and it’s great that he’s committed himself to Pompey,” Pompey chief executive Peter Storrie told the club’s official website.
“Papa wanted to extend his contract last season but he injured his knee so first we wanted to make sure he was fit and there were no problems.”