Everton have completed the transfer of Allan on a three-year-deal.
Allan will reunite with Carlo Ancelotti at Everton, having previously worked under the Italian for two seasons while he was in charge of Napoli.
That relationship between player and manager is said to have ensured Everton beat off competition from Atletico Madrid and Borussia Dortmund for the 29-year-old midfielder.
The Brazilian international joined Napoli from Udinese in 2015 and made over 200 appearances for the club, also winning the Coppa Italia last season.
Allan: “It is a real pleasure to sign for Everton. I am immensely happy to be here,” Allan told evertontv. “I hope, like I have done in my entire career, I contribute with my performances together with my teammates and that I put in some great games, great performances and win important things.
“It is a club with a rich history in the Premier League, has real ambition and then there is Professor Ancelotti.
“He has done everything possible to bring me here.
“It is the size of the Club and the name of the coach which means you don’t think twice about coming to Everton.
“A strong part of my game is marking and the desire going in to win absolutely every ball,” he explained.
“I believe with my desire I can put in really good performances to win the fans over and I hope to win things as well. There will be a tremendous amount of dedication from me, a lot of dedication – a lot.
“I received a lot of messages from Everton fans and that made me really happy and confident in the work I can do here.”