Pompey boss Steve Cotterill is impressed by Riise's versatility, with the 28-year-old able to play on the right or left of defence, as well as in midfield.
"He's a central midfielder who can also play on the right side," Cotterill told his club's website.
"He's energetic, a good footballer and has played 31 times for his country. He played on loan for Sheffield United against us last season and did very well.
"We've got several midweek games and we may need to shuffle the pack so he will help us out."