Matt Clarke has extended his Pompey contract until the summer of 2020.
The centre-back has scored five goals in 101 appearances for the club since signing – initially on loan – from Ipswich in 2015.
He was an integral part of the side that secured the League Two title last term and has continued to be a regular in the third tier.
Blues boss Kenny Jackett said: “Matt’s one of many promising youngsters at the club and so we’re pleased that he’s committed his future to us.
“We’re looking to shape a side that can get into the Championship and he’s the type of player that you can build things around.”
Clarke added: “Every player just wants to be at a good club and so I’m really pleased to have signed a new deal.
“I originally arrived with the sole mission of helping get Pompey out of League Two. We’ve done that now, but want to continue that progression.
"Every player just wants to be at a good club and so I’m really pleased to have signed a new deal."
“You are only going to improve with minutes on the pitch and that’s exactly what I’m getting here at Fratton Park.”
Clarke scored the crucial equaliser in Saturday’s victory at MK Dons and also netted the winning goal against Bury back in December.