Mertens is new Napoli manager Rafael Benitez's first signing at the San Paolo Stadium, with the winger putting pen to paper on a five-year contract in Naples.
The 26-year-old declared that Benitez's insistence on closing a deal quickly has given him confidence as he prepares for a debut campaign in Serie A.
He told De Telegraaf: "Rafael Benitez wanted me really badly and that gives me a lot of confidence.
"And when people doubt me and say Serie A is different from the Eredivisie because I won't play against any Dutch defenders, that is when I'll be at my best."
Napoli president Aurelio de Laurentiis also used Twitter to confirm the move, sharing his delight at acquiring a long-term transfer target.
He tweeted: "I am happy to tell you that a few minutes ago I closed the deal with PSV for Dries Mertens.
"In the last few days Dries has been in Naples. He told me that he fell in love with our city, with its greatness. Welcome Dries!"
The Belgium international has 19 caps for his country and scored 16 goals for PSV last season.