The Liverpool goalkeeper has signed on a season-long loan.
"Napoli are the best solution for me. I will give my all this season, then we'll see. I am playing for a great club that will fight to win the Scudetto and impress in the Champions League. I had been missing both those elements," said Reina.
"We are building a fine squad that will go back to the top. Benitez is a guarantee, as he's a great Coach who was able to get the best out of me. He's the best manager I worked with, the city is beautiful and I'd perhaps have come to Napoli anyway, but Rafa's presence made it easier."