After last season's loan with Alaves, Marin has cut ties with Real Madrid to sign with the Azzurri.
Taking to social media, Marin posted: "I arrived in Valdebebas as a 14-year-old boy, willing to sacrifice everything to be a soccer player and with a backpack full of dreams. In this time, I have achieved some objectives, but there are many others left to conquer. This is just the beginning and we have to continue fighting for them, until the end, as they have taught me at Real Madrid.
"Being part of and growing within the Madrid family is something unique, which remains within me forever.
"Now, it's time to separate our paths to continue moving forward with everything, with more ambition and more strength than ever. Playing for Naples is a source of pride, another dream, and it has to do with the work done, also at Alavés.
"Thanks to everyone who is part of Real Madrid and has helped me in this process, thanks to their love for the support and affection that they have always shown me, also in these moments. You are incredible. Here I have been trained as a man and as a footballer. I take with me the values: humility, respect, the importance of a job well done and the winning character, which are the best legacy of this club.
"GO MADRID!"