The Naples outfit have had to bide their time to bring the 24-year-old to Stadio San Paolo, but got their man on Tuesday.
Defensive midfielder Allan was pictured with Napoli president Aurelio De Laurentiis on the club's Twitter feed, along with confirmation that the player had passed a medical and signed a contract.
No further details of the deal for the former Vasco de Gama player were disclosed.
Allan joined Udinese from Vasco in his homeland three years ago and went on to become a first-team regular at the Stadio Friuli.
