Parma president Tommaso Ghirardi has confirmed the deal with the Greece international.
"It's true, we have got Ninis, who is one of the best young players in Europe and we are proud he has chosen us!" Ghirardi told TVParma.
"His contract was set to expire and he could've joined a bigger club, but he preferred Parma. Don't forget this lad was in the last World Cup and is considered one of the top prospects on the continent."