Craig MacGillivray has extended his Pompey contract until the summer of 2021.
The 26-year-old goalkeeper has been an ever-present in the league and FA Cup since signing from Shrewsbury last summer.
He has made 30 appearances so far and kept 11 clean sheets to help the Blues climb to the top of the table.
MacGillivray said: “I’m obviously delighted to have signed a new contract and committed my future to the club – I’m here for a bit longer yet.
“The first half of the season has been a whirlwind and it’s nice to be playing games. Hopefully we can get back to winning ways on Saturday.
"I’m obviously delighted to have signed a new contract and committed my future to the club – I’m here for a bit longer yet."
“Our defence has been superb and I like to think I’ve been pretty consistent, which is hopefully something that will continue.
“I’m very settled here – and it’s certainly lot, lot warmer down south! I’ve met my girlfriend and really love the area.”
Blues boss Kenny Jackett added: “We’re delighted with what Craig has brought to the team this season – both as a goalkeeper and as a person.
“It’s still very early in his Pompey career, but we wanted to reward him and give him a longer-term deal.
“He’s been a very good addition to our squad and I’m sure will continue to prove that in the years to come.”