Pompey have completed the signing of Scottish centre-back Clark Robertson on a two-year deal.
The 27-year-old will officially become a Blues player when his Rotherham contract expires at the end of the month.
Head coach Danny Cowley said: “We’re delighted to sign Clark, who brings with him a wealth of Championship experience.
“He has also been promoted from this division and, at the age of 27, is coming into the prime of his career.
“It was very important for us to sign a left-sided centre-half in order to provide balance in that area of the pitch.
"Clark is a really good footballer and also a really good person. I believe he will add to the character in our dressing room."
“His playing characteristics fit with our game idea, while he possesses a real composure on the ball and a nice range of passing. In that sense, as well as being our first senior defensive signing, he’s also our first attacking signing!
“Clark is a really good footballer and also a really good person. I believe he will add to the character in our dressing room.”
Former Scotland under-21 international Robertson began his career with Aberdeen before moving south of the border to join Blackpool in 2015.
Robertson made more than a century of appearances during his time at Bloomfield Road and netted in a victory at Fratton Park.
He was part of the side that won the League Two play-off final in May 2017, playing the full 90 minutes of the Wembley success over Exeter.
Robertson signed for Rotherham in 2018 and helped them secure promotion to the Championship, scoring five goals in 69 games for the Millers.