Napoli president and film producer Aurelio De Laurentiis made the announcement as promised on Twitter, calling Benitez "a man of great international experience. A leader."
Benitez is coming off a spell as Chelsea's interim manager, having led the London club to the Europa League title earlier this month. The Spaniard formerly coached Inter Milan in Serie A for the first half of 2010-11.
Walter Mazzarri resigned as Napoli coach earlier this month and signed with Inter.
Napoli finished second in Serie A to qualify for next season's Champions League group phase.
Benitez's first order of business with Napoli will be to convince star forward Edinson Cavani to stay. Cavani led Serie A with 29 goals but appears interested in a transfer.
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