The 29-year-old is the first of three midfielders expected to arrive at Goodison Park this summer.
Allan has signed a three-year deal with Everton, moving to England after five years with Napoli.
— Everton (@Everton) September 5, 2020
He played 61 games under current Toffees boss Carlo Ancelotti when the Italian was in charge of Napoli.
Ancelotti’s presence on the Everton bench played a key part in attracting the Brazilian to Merseyside.
“It is a real pleasure to sign for Everton. I am immensely happy to be here,” Allan said upon signing. “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.”
Borussia Dortmund and Atletico Madrid had also expressed an interest in signing Allan this summer.
Last year, Paris Saint-Germain had a €50m bid rejected by Napoli for the tenacious midfielder.