The Spaniard will join the Italian team for the coming season.
Callejon signed a four-year deal with the Serie A club, bringing his second stint with his boyhood club, Real Madrid, to a close.
The 26-year-old made 40 appearances under Jose Mourinho at the Santiago Bernabeu last term, but will now hope to help his new club build on their second-place finish in the Italian top flight.
Callejon began his career with los Blancos’ C and B teams, after graduating from the club’s youth academy, but after growing frustrated at a lack of first-team opportunities in the Spanish capital, he moved on to Espanyol in 2008.
His time at the Catalan club was the most successful of his career up until that point, as he made 106 appearances over three seasons, scoring 12 goals.
That form convinced Madrid to bring him back to the club for a fee of €5.5m, and the forward, who plays from the wings, would go on to play a prominent role in their 2011-12 title-winning campaign.
He, however, played a limited role for Madrid in the 2012/2013 season partly due to injury, and also loss of form.
He has now opted to take on a new challenge, however, and will link up with Napoli as they begin life under new boss, Rafael Benitez.
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