Alex Bass has put pen to paper on a new three-and-a-half-year Pompey contact.
The deal will keep the 21-year-old goalkeeper at Fratton Park until the summer of 2023.
Bass has appeared 13 times for Blues since graduating from the academy, making his debut in a League Cup tie at Coventry in 2016.
He has already featured nine times during the current campaign, including in last weekend’s victory over AFC Wimbledon.
“I’m delighted to be here for such a long time and it came out of the blue,” said Bass. “I certainly wasn’t expecting it.
“The league matches are completely different and there’s extra pressure on them. I’ve thoroughly enjoyed the ones I’ve been involved in and there are hopefully many more to come.
"It’s been a good season for me, although it’s just the start and now I’m really looking to kick on in the second half."
“I thought it would be another season of playing in the Leasing.com Trophy games and gaining my experience from them.
“So I didn’t expect to get the opportunity I’ve been given, but I’m obviously very happy that I have and it’s an honour.
“It’s a massive privilege to be in the team ahead of someone like Craig MacGillivray, who’s a very good keeper and has been away with Scotland.
“We work hard together as a unit and it just goes to show the strength in depth we’ve got in that department.
“It’s been a good season for me, although it’s just the start and now I’m really looking to kick on in the second half.
“There are always areas where you can improve and I’m still only young, so I’m keen to continue developing.”