The 25-year-old defender arrives at Fratton Park on a 2-year-deal following his departure from Lincoln City this summer.
Regan Poole, who becomes the Blues' 10th signing of this summer transfer window, left Lincoln City by mutual consent at the end of last season after both player and club failed to agree terms for a new contract at Sincil Bank.
The Cardiff-born player is known to have played at right-back for the majority of his career to date, but can also play in the centre of the defence.
He has represented Wales at youth level on 35 occasions, coming through the ranks on the international stage all the way through from the Under-17s to the Under-21s, for whom he featured 23 times between 2016-2020.
Reliable sports broadcaster and Pompey commentator for BBC Radio Solent, Andrew Moon, believes the arrival of Poole, as well as the additions of Conor Shaughnessy and Jack Sparkes to Portsmouth's squad, makes a move for Jamaican international Di'Shon Bernard "much less likely".
This move makes it much less likely that #Pompey will sign DiShon Bernard— Andrew Moon (@mrandrewmoon) July 10, 2023
John Mousinho said: "Regan brings a lot of league experience, having played at a consistently high level for Lincoln over the past few years.
"That included taking them all the way to the Play-Off Final, while he has also worn the captain's armband.
"He's a versatile defender and is very good on the ball, while being aggressive off it and has excellent qualities.
"In terms of the all-round package of playing and leadership, we're getting someone who's very exciting.
"We probably won't see him play in the friendlies this week, but he'll come in and train with us straight away.
"We were thrilled to take nine new players away with us to Spain last week and it's important that we get these signings in as soon as possible to get them used to the way we want to do things."
Poole will wear the number 5 shirt at Fratton Park.