Gerard Deulofeu has signed a three-year-contract at Goodison Park with a further option for another year.
The winger, who completed a £4.2m move to Everton from Barcelona, had already spent the 2013-14 season on loan at Goodison Park, scoring four goals in 11 starts for the club.
The Toffees secured the move yesterday and Deulofeu's deal includes a release clause if Barcelona match Everton's requirements within the first two seasons.
The La Liga side also have a right of first refusal if the Merseysiders decide to sell the 21-year-old.
“Gerard needs no introduction to anyone here at Everton, for we are all well aware of his incredible footballing talent,” said Everton manager Roberto Martinez.
“We got to know him really well from his loan period, not only as such a talented player but also his great character. He will be a fantastic asset to our Club.”