The 25-year-old French-born midfielder arrives at Fratton Park for an undisclosed fee despite reported interest from other clubs.
"I watched Nigel play on numerous occasions for the Hawks last season and was impressed with what I saw," Pompey boss Andy Awford told the club's official website.
"I believe he's capable of making the step up and playing in the Football League.
"It's great that Nigel's chosen to come and play for Pompey and I look forward to working with him."
Atangana becomes the south-coast club's fourth summer-signing, following in the footsteps of Craig Westcarr, Danny Hollands and Paul Jones.