The 27-year-old free agent became Pompey's second signing of a busy day in the transfer market when he put pen to paper on a three-year deal at Fratton Park, with the defender to become an official Blues player when his contract at Blackburn expires at the end of the month.
The centre-half quickly followed Ellis Harrison's move from Ipswich to Fratton Park for an undisclosed fee, believed to be around £450,000.
And his arrival follows hot on the heels of Matt Clarke leaving for Premier League Brighton, with the centre-half signing a four-year deal at the Amex for a fee which is also undisclosed.
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Clarke's departure will no doubt be felt as Kenny Jackett bids to return the Blues to the Championship.
However, assistant manager Gallen believes Pompey have swooped for a player that will seriously cushion that blow.
Downing has experience of getting out of League One, following Blackburn's promotion in 2018.
He also played an important role during his loan at Doncaster Rovers in the second half of last season. helping Donny to the play-offs.
And with the Blues eager to improve on a clean sheet record of 12 in the league last season - including the play-off against Sunderland - Gallen believes Pompey have found the man to help him achieve their objectives.
Gallen said: 'Paul’s an experienced centre-half and, like with most of our signings, we’ve been observing his performances for a while.
'He brings a lot of knowledge and leadership qualities to our back four, so we’re delighted to get this deal over the line.
'We’ve got some decent players in that department and it will be a good battle, but he’s someone who knows how to get out of this division.
'We have excellent defenders in our squad, but the fact remains that we wanted to be higher up the clean sheet scoreboard and he can help us achieve that.'