The 37-year-old has come out of retirement and have joined Pompey on a free transfer.
“Antti has got vast experience and will be a welcome addition to the squad,” Pompey boss Paul Hart told the club’s official website.
“Our goalkeeping coach David Coles worked with him in the past and told me all about his character and his quality.
“He has a massive appetite to succeed and he relishes a challenge.
“It provides David James with some good competition and also allows Asmir Begovic to go out on loan, while Jamie Ashdown needs first-team football as well.”
