Jack Whatmough has signed a new deal to keep him at Pompey until the summer of 2021.
The 22-year-old centre-back has been a regular in the Blues back-line this season, ensuring the club currently sit at the League One summit.
He follows in the footsteps of goalkeeper Craig MacGillivray, who extended his Fratton Park deal earlier in the week.
Whatmough said: “I’m delighted to be here for another two-and-a-half years. This is a good squad to be a part of and I’m looking forward to the future.
“From the minute I signed here as a 12-year-old, I’ve always felt wanted. There’s never been any question about the club’s loyalty to me or from me to them.
"From the minute I signed here as a 12-year-old, I’ve always felt wanted."
“I’ve had setbacks in my career with injuries, but I’m enjoying my football now and don’t really want to look back.
“You only get rewarded with a contract if you’re doing your job, so now I want to stay in the team and keep performing well.
“I want to be playing as high as I possibly can – and hopefully that will be in the Championship next season.
Whatmough has made 92 appearances for the first team, having progressed through the Pompey academy.
He was part of the squad that dramatically secured the League Two title in 2016/17, when he netted his only senior goal to date at Carlisle.