The deal will be some consolation for the FA Cup winners following the FA Community Shield penalty shoot-out defeat to Manchester United.
Kaboul, 22, will sign subject to passing a medical at Fratton Park on Monday, with the fee thought to be around £6million for a player who recently turned down Sunderland and was also linked with Aston Villa.
Coach Joe Jordan said: "Kaboul should help. He's a been a long-term target of the manager. He's only 22 and is coming to a good place to learn, with Sol Campbell and Sylvain Distin."