Everton have signed Brazilian midfielder Allan from Napoli on a three-year contract for a fee believed to be around £21.7m.
Allan is Everton's first signing of the summer and the move reunites him with former Napoli boss Carlo Ancelotti.
"It is a real pleasure to sign for Everton. I am immensely happy to be here," said the 29-year-old.
Everton are also closing on deals for Watford's Abdoulaye Doucoure and James Rodriguez from Real Madrid.
Allan played 61 games under Ancelotti at Napoli, and he says the Italian did "everything possible" to bring him to Goodison Park.
Allan added: "I hope, like I have done in my entire career, I contribute with my performances together with my team-mates 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.
"It is the size of the club and the name of the coach which means you don't think twice about coming to Everton."
Allan made more than 200 appearances for Napoli after joining from Udinese, and played alongside Toffees forward Richarlison as hosts Brazil won the 2019 Copa America.
"A strong part of my game is marking and the desire going in to win absolutely every ball," he said.
"I believe with my desire I can put in really good performances to win the fans over. I received a lot of messages from Everton fans and that made me really happy and confident in the work I can do here."
Ancelotti's side travel to Tottenham for their opening Premier League game on Sunday, 13 September (16:30 BST).
BBC chief football writer Phil McNulty
Allan has been a top priority signing for Everton manager Carlo Ancelotti once he decided he had to reshape his midfield.
The Brazil international is a tough-tackling, mobile competitor, the sort Everton have lacked since the departure of Idrissa Gueye to Paris St-Germain and in the absence of his permanently injured successor Jean-Philippe Gbamin.
Ancelotti admires Allen hugely having worked with him at Napoli and the feeling is mutual, with the player referring to his manager as 'The Professor'.
Everton's midfield was simply too easy to play against last season and Ancelotti is determined to address that flaw with signings such as Allan, who says "winning a 50-50 is like scoring a goal", and Watford's Adboulaye Doucoure.
It is the first serious business concluded by Ancelotti - and one he regards as vital.
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