The former Chelsea interim boss signed a two-year deal and will replace Walter Mazzarri, who has moved to Inter Milan.
"Now that the agreement is confirmed, I can say I am very happy and very satisfied for having committed to a great club with an illustrious heritage like Napoli," he said on his official website, www.rafabenitez.com.
"I understand the history of the city and how the club forms a fundamental part of the culture and traditions of Naples.
"I must confess that I am extremely excited because I can share my passion for football with the fans of Napoli; it is proven that they are very, very special."
Benitez led the Blues to third place in the Barclays Premier League and won the Europa League after he was appointed manager in November, following the sacking of Roberto Di Matteo.
Despite his success, the Spaniard struggled to win over a section of the Chelsea supporters during his time in charge at Stamford Bridge. Benitez is hopeful of a better relationship with the Napoli fans and is confident of bringing further success to a side who were second last season - their best finish in Serie A since winning the Scudetto in 1990.
Benitez also confirmed that fitness coach Francisco de Miguel Moreno and goalkeeping coach Vicente Javier Valero Berchili, who were both part of his backroom team at Liverpool, Inter Milan and Chelsea will join him once again.
Benitez will begin his new job in July and Napoli were delighted with their acquisition.
"Rafa Benitez is the new coach of Napoli," read a statement on www.sscnapoli.it. "The agreement was signed today in London. SSC Napoli is happy to welcome a coach so prestigious in their own family. A welcome also to his collaborators."