Gareth Evans has extended his Pompey contract until the summer of 2021.
The 30-year-old midfielder has been a regular fixture in a side that is pushing for promotion from League One.
He has netted 12 goals in 45 appearances so far this term, including the opening goal in last month’s victory at Bury that secured a spot at Wembley.
Evans said: “I’m delighted to sign and extend my stay on the south coast. The gaffer’s shown a massive amount of faith in me and I think I’ve repaid him with my performances.
“I believe we can get promoted to the Championship this season and I’m sure that, in years to come, this club will be back in the Premier League where it belongs.
"I’m delighted to sign and extend my stay on the south coast."
“The reason I first came here was to help Pompey achieve success and I was sure that it could be done.”
Blues boss Kenny Jackett added: “I’m delighted that we’ve got these contract negotiations over the line and Gareth has committed his future to the club.
“He’s very much a valued member of the team and in a young squad, the experience he offers is vital for us.”
Evans arrived at Fratton Park in the summer of 2015 and helped the club dramatically seal the League Two title a couple of years later.
The former Manchester United trainee – who later appeared for Macclesfield, Bradford, Rotherham and Fleetwood – has netted 31 times in 176 games for the Blues.