The former Bourdeaux star joined on a free transfer, after being released by Auxerre at the end of last season, and has signed a one-year contract.
The midfielder, who has experience of playing in the Champions League, also had trials at Blackburn, Charlton and Middlesbrough over the summer but Pompey is the only club to offer him a deal.
Redknapp described Diabate as: "A quality player who has played at the highest level," but refused to comment on whether his signature brings to an end Portsmouth's interest in Tim Sherwood.