The 26-year-old has been announced as Pompey's 6th summer signing as John Mousinho continues to bolster his squad ahead of the new campaign.
Ben Stevenson, who plays in midfield, left Forest Green Rovers following the expiration of his contract at The New Lawn this summer.
He arrives on the South Coast having signed a 2-year deal, with there being an option for both parties to extend by a further 12 months.
It was against Pompey where Stevenson made his senior debut in football, that a Carabao Cup victory for the Sky Blues in 2016, a year after he signed his first professional contract.
The Blues' head coach, John Mousinho said: "We're bringing in a player who covers all positions in the middle of the park.
"He's a really good footballer and an excellent character as well, so we're delighted to have him at Pompey.
"Ben will be looking to add a few more goals and assists to his game, but he's got all the attributes we're looking for.
"I think he compliments the guys we already have in the building and has an important role to play off the pitch, as well as on it."
The Leicester-born midfielder was poached by Wolverhampton Wanderers in 2018 but failed to make a competitive appearance for the club, instead being shipped out on loan spells to both Colchester United and Spanish lower-league outfit Jumilla.
Between 2019 and 2021, Stevenson represented Colchester United before moving to Forest Green, where he would go on to make 79 appearances in all competitions, scoring 2 and contributing to 5 assists.