The 21-year-old forward arrives for an undisclosed fee from Premier League side AFC Bournemouth.
Christian Saydee has this evening become Pompey's 3rd signing of the day, the first day of the summer transfer window.
The former Poole Town, Weymouth, Burton Albion and Shrewsbury Town loanee arrives at Fratton Park on a permanent deal, signing a 3-year deal with the Blues.
Saydee, born in Hillingdon in 2002, had a productive spell at Shrewsbury Town last season.
The attacker graduated from Bournemouth's academy in 2019 and since made several appearances in the Championship, though it's his seven goals from 38 appearances for the Shrews last time out which has caught the eyes of those at PO4.
Boss John Mousinho told club media: "Christian is an exciting young prospect and arrives here off the back of a successful spell in League One.
"His tally of goals for Shrewsbury was very impressive for someone his age and we believe this is the next step for him.
"We feel he can really flourish here and he's an exciting prospect for the future, but can also do some damage now.
"Christian is a powerful and athletic forward who can run the channels and is also excellent in front of goal." - Concluded Mousinho.
Saydee had a short period on trial with Portsmouth last summer, running out for the Blues in friendlies at Havant & Waterlooville and Gosport Borough, though a move never materialised.