Winger Jerome Thomas has agreed a one-year contract with Portsmouth, who have decided to make his loan move from Charlton permanent.
Pompey will pay Charlton an undisclosed fee if the 25-year-old, who joined last week and made his debut as a substitute in the 4-0 Barclays Premier League defeat at Chelsea, makes a specified number of appearances.
Chief executive Peter Storrie said: "Jerome could have been recalled by Charlton in January under the terms of his loan, but that's no longer a factor now."
Under Premier League rules, clubs are allowed only four loans per season - and Portsmouth already have striker Ben Sahar on a six-month stay from Chelsea.
The league have also confirmed no club is allowed to have more than two players from any one other on loan at the same time.
That rules out any immediate Pompey loan move for England winger Shaun Wright-Phillips, who is also at Chelsea.