Rafa Benitez's side agreed undisclosed terms for the Espanyol academy graduate on Saturday, but they have now formally confirmed Lopez as a Napoli player.
"I'm happy to be in Naples, an important club with a wonderful audience," he told the club's official website www.sscnapoli.it.
Napoli club president Aurelio De Laurentiis also used Twitter to make his new signing feel at home, saying: "Welcome to Naples David Lopez."
The 24-year-old holding midfielder made 33 Primera Division appearances in his breakthrough season at the Power8 Stadium last year.
Before that he found first-team opportunities hard to come by at Espanyol and spent two seasons away from the club on loan at Leganes and Huesca.
Napoli get their 2014-15 campaign under way at Genoa on Sunday, but Lopez will have to wait until after the international break for his first taste of Serie A action.