The 26-year-old forward joined the Serie A club from PSV Eindhoven for 9.5million euros after two years with the Eredivisie side.
The Belgian was presented to the press and revealed why he had joined Rafael Benitez's team.
He told reporters: "Napoli is a choice from my heart.
"I think Napoli is a very strong team and I hope I can repay the confidence the club has in me.
"I know Serie A is a tough competition. I will do everything to adapt to Italian football and the language.
"It's important for me to come to play but the results of Napoli override everything.
"We have a strong selection of great footballers. Everyone should act in the interest of the club, "said the winger, who is looking forward to next season's Champions League.
"The Champions League is also a reason why I came to Naples. It will be exciting to play at that level," said Mertens.