Pompey have agreed a deal for left-back Lee Brown.
The 27-year-old has signed a two-year contract with the Blues.
Brown arrives from League One rivals Bristol Rovers after turning down a new deal at the Memorial Stadium.
The England C international made 319 appearances for the Gas over the past seven campaigns, scoring 22 goals.
The defender remained with Rovers when they were relegated to the National League in 2014, before helping his side achieve back-to-back promotions to League One.