Adam May has signed a two-year Pompey contract.
The new deal will keep the 20-year-old midfielder at Fratton Park until the summer of 2020.
May is a graduate of the Blues academy and has made a total of 25 appearances for the first team.
He said: “I’m delighted to have signed a new contract with a club that I have been at since a very young age.
“When you’re young, confidence comes with getting a run of games. That’s what I got last season and it can only make you a better player.
"The gaffer is a really impressive manager with experience at a higher level, so it means a lot that he’s put his trust in me."
“There will be good and bad days – and you have to get through the latter because what’s ahead at the end of the tunnel is worth it.
“Hopefully I will get more of the good ones next season because this is a great club and it’s exciting to see what the future holds.
“The gaffer is a really impressive manager with experience at a higher level, so it means a lot that he’s put his trust in me.”
May was just 17 when handed his senior debut by caretaker boss Gary Waddock in a home defeat to Bury at the end of the 2014/15 campaign.
He made a handful of appearances over the course of the next two campaigns, while also gaining experience with spells on loan at Gosport and Sutton.
The latter included a memorable run to the fifth round of the FA Cup and a clash with Premier League giants Arsenal.
But May enjoyed a breakthrough season last term, playing 18 times and almost opening his goalscoring account with a stunning strike at Fleetwood that hit the woodwork.