Pompey have signed defender Paul Downing on a three-year deal.
The 27-year-old – who spent time on loan at Doncaster last term- will officially become a Blues player on July 1 when his Blackburn contract expires.
Assistant manager Joe 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.
"Paul’s an experienced centre-half and, like with most of our signings, we’ve been observing his performances for a while."
“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.”
Downing started his career with West Brom, but failed to make a senior appearance and moved to Walsall in 2012.
He made almost 200 appearances during four years with the Saddlers, then had a stint with MK Dons before, in the summer of 2017, signing for Blackburn.
The centre-back was part of the Rovers side that won League One promotion in 2017/18 and played 22 times while on loan with Doncaster in the second half of last season.