Paul Downing admitted Pompey 'ticked all the boxes' for him after he agreed a three-year deal at Fratton Park.
The central defender signed for the Blues on Friday, with his move to be officially confirmed upon the expiry of his Blackburn contract at the end of the month.
Kenny Jackett, who also added Ellis Harrison to his ranks hours earlier, saw off stiff competition from League One rivals Doncaster Rovers to land the 27-year-old.
The Blues boss is desperate to lead Pompey to the Championship and sees Downing as a key part of the jigsaw, particularly in the aftermath of Matt Clarke's Brighton departure.
That promotion determination, coupled with the infrastructure in already place at Pompey, proved huge in Downing's choice to move to the south coast.
And he revealed it was the overriding factor in his decision to turn down a return to Rovers, for whom he played 22 games on loan during the second half of last season.
Asked why he opted to join Pompey, Downing said: 'I think it was the size of the football club and the ambition it has got.
'Obviously, you play at Fratton Park as an away player and you see the atmosphere and the size of the club.
'That was a big pull for me.
'You think the chance it's got to get out to the division and play in Championship.
'I'm coming to the time of my career where I want to play as high as possible so it ticked a lot of boxes for me.'
The former West Brom trainee played four times for Blackburn in last season's Championship, before moving to the Keepmoat on loan.
That was after playing key role in Rovers' 2017-18 League One promotion campaign, amassing 30 appearances.
Downing was pleased with the impact he made at Donny, helping them reach the play-off semi-finals.
However, he's looking forward to another new challenge and helping Pompey achieve their promotion ambitions.
'I needed to go and play football after a frustrating first six months to last season,' added Downing, when asked about his time at Doncaster.
'I went there and managed to get into the team, which is not always easy because they were doing well and probably overachieving.
'I knew I had to go there and hit the ground running, which I did.
'I managed to put in some good performances and help them get into the play-offs.
'I enjoyed my time at Doncaster but definitely now I'm looking forward to this season and ready to crack on.'