The 18-year-old, who was let go by the Blues this month following the expiry of his third-year scholarship, will link up with former Pompey coach Scott Green at the Rauma-based side.
Green has been in charge at the Äijänsuon Stadion since October 2018 and has returned to his former employers in a bid to boost his side’s current seventh place in Kakkonen group B.
Dandy, who can play at either centre-back or right-back, skippered the Blues’ academy last campaign and was named the Scholar of the Year at the club’s end-of-season gala dinner.
Having suffered a long-term knee injury towards the back end of last term, the Ascot-born teenager was invited back to Fratton Park in the summer to get fit.
He then penned a contract until January and had a spell on loan at Bognor.
Dandy also went on trial with National League side Aldershot last month.
He’s expected to link up with his new club on Febraury 12.