Gary Roberts has penned a three-year deal with Pompey.
The midfielder has joined the Blues after Chesterfield accepted an undisclosed bid for the 31-year-old.
His signing brings an end to new manager Paul Cook’s pursuit of a player he identified as key to his Fratton Park plans.
Roberts is the fourth summer signing made by Cook, following his appointment last month.
Left-back Enda Stevens arrived earlier this week on a free transfer, after last month’s capture of out-of-contract duo Kyle Bennett and Kal Naismith.
Cook told the Pompey website: ‘I’m delighted that we’ve signed Gary and I view this as a no-risk transfer, which is always great.
‘His record speaks for itself and I’m sure the fans will be looking forward to seeing him in action.’