Pompey have signed Dion Donohue from Chesterfield for an undisclosed fee.
The 23-year-old Welshman – who can play in midfield or at the back – has penned a two-year deal, with the club holding an option for a further year.
He will wear the number 17 shirt, but has not signed in time to feature in Saturday’s League One clash with Walsall at Fratton Park.
Blues boss Kenny Jackett said: “We have brought in a bright and energetic young player. He has a very good left-foot and adds versatility to our squad.”
"We have brought in a bright and energetic young player."
Donohue was a member of Everton’s academy and later appeared for Porthmadog, Holyhead, Caernarfon and Sutton Coldfield.
He signed for Chesterfield following a successful spell on trial in 2015 and scored once in 65 appearances.
Donohue has yet to be capped by Wales, but did represent Anglesey at the 2011 Island Games.