Pompey will pay Charlton a small fee for the winger if he makes enough appearances this season but the main reason for this deal is to free up a space of Pompey's loan quota.
Premier League clubs are only allowed to make two loan signings and having already borrowed Ben Sahar from Chelsea, Harry was unable to loan in any more players until Thomas made his move permanent.
It is reported that Harry will use the extra loan to bring in Arsenal left back Armand Traore on a season-long loan. The 18-year-old Frenchman is down the pecking order at Arsenal after Arsene Wenger brought Mikael Silvestre to the club.