The 26-year-old centre-back has agreed a two-year deal and the move will go through when his contract at the County Ground expires on July 1.
Devera told his new club's official website: "Pompey had problems and were in administration last season. But they did well under the circumstances and finished the season strongly.
"I don't really look at coming here as dropping down a league because they are a really big club and can fight their way back.
"Pompey can definitely make a big impact in League Two, although there is a lot of hard work to do because we will be starting with a lot of new players.
"I can't wait to get started and am already looking forward to coming down for pre-season and also moving to the area."