Brandon Haunstrup has put pen to paper on a two-year Pompey contract.
The new deal keeps the 21-year-old left-back at Fratton Park until the summer of 2020.
Haunstrup is a graduate of the Blues academy and has made a total of 26 appearances for the first team.
He said: “It’s good to get this out of the way, so I can start concentrating on next season and trying to get a spot in the side.
“I had a decent run of games last season and then I unfortunately got injured, which was frustrating for me.
"Hopefully I can be part of a winning squad that gets up the table and wins promotion."
“But the manager spoke to me last month. He told me that he wanted me to stay at Pompey and really push on.
“Hopefully I can be part of a winning squad that gets up the table and wins promotion. I’m sure the new lads we’ve got will be great for the club.”
Haunstrup made his Pompey debut in a memorable League Cup victory over Derby at Fratton Park in August 2015.
He also featured against Reading and Ipswich that season, before gaining more experience with a loan spell at Sutton the following campaign.
But last term was Haunstrup’s most productive in a Blues shirt, with the youngster featuring 20 times as Kenny Jackett’s side finished eighth in League One.