John Sullivan has completed a switch to Fratton Park.
The goalkeeper, who is out-of-contract at Charlton, becomes the 10th player to commit their future to Pompey.
Aged 25, he enjoyed a successful loan spell at AFC Wimbledon at the end of the season.
He has also represented Brighton, Colchester and Yeovil.
Sullivan passed a medical this morning ahead of signing a contract and will compete with Phil Smith for a first-team spot after Simon Eastwood turned down a deal to instead join Blackburn.