The 27-year-old, an ever-present for Crawley last season, was available on a free transfer after leaving the League One side at the end of his contract.
Blues boss Andy Awford told the club's official website: "We're delighted he signed - it's a great acquisition for us.
"I watched him play for Crawley a few times last season and I liked what I saw. There have also been good reports from other people in the game that I trust.
"We fought off stiff competition from other clubs to bring him to Pompey and it's great that he chose us."
Awford has also confirmed that defender Bondz N'Gala will not be offered a new deal and will depart having made 31 appearances in his sole season with Pompey.