Marin made just six appearances for Napoli during their title-winning season last year. Despite this, he has gained experience playing for a top-level team.
The transfer deal involves an initial €1m loan fee for Marin, who is a Spain U21 international. He was signed by Napoli from Real Madrid for €12m a year ago.
Villarreal is excited to have Marin on board, and fans are looking forward to seeing him in action.