The Spanish U21 player Deulofeu, 21, has signed a three-year contract.
He spent the 2013/14 season on loan at Everton, and was deemed a huge success, making 25 appearances and scoring three goals as Roberto Martinez's side finished fifth in the table to qualify for the Europa League.
"I'm back! Once a blue, always a blue!," tweeted Deulofeu about his return to Goodison Park.
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"Gerard needs no introduction to anyone here at Everton. We are all well aware of his incredible footballing talent," said Martinez.
"Gerard considers Everton as home and we are getting an even more mature player now than he was a couple of seasons ago."
Deulofeu turned out 28 times for Sevilla in Liga on loan last term and helped them win the Europa League, but the move was viewed as a disappointment compared to his previous season in England.
He becomes Everton's second signing of the summer after Manchester United and England midfielder Tom Cleverley on a free transfer. Both players officially join on July 1.
“It was always our intention to work extremely hard behind the scenes to get some really good business done early on during the transfer window,” said Martinez.
“Our objective was to secure Tom and Gerard as quickly as we could and I’m really pleased this is exactly what we’ve done.
“Having done this now, we will have more time over the remainder of the window to carry on strengthening our squad, which is already strong, and we will use this time ahead of us well.”

Gerard Deulofeu against Real Sociedad with FC Sevilla during Liga game 24-09-2014

Image credit: AFP

Barcelona confirmed the transfer in a statement with the caveat that they have the first option to buy the player back.
"FC Barcelona and Everton FC have reached an agreement for the transfer of midfielder Gerard Deulofeu," said Barcelona in a statement.
"The contract includes a release clause if Barcelona make an offer to Everton meeting certain requirements. FC Barcelona also have a right of first refusal if Everton decide to sell the player.
"FC Barcelona's Management Commission approved the transfer after receiving a positive report from the club's Technical Committee."
OUR VIEW
Everton will be hoping to sign the exciting attacking force that sparkled for them two seasons ago. Interestingly enough, Deulofeu was recently named in the Spanish newspaper Marca's worst side of the season as he struggled to replicate his performance levels at Goodison Park with Europa League winners Sevilla. Which suggests he has a point to prove to himself, Barca and Sevilla. A player hungry for success can't be a bad one.
Deulofeu scored one exceptional goal at Arsenal in a 1-1 draw in the Premier League 18 months ago, and it is easy to forget he is only 21. Little wonder Barca are protecting themselves with the buy-back clause inserted in the transfer in case his career really begins to take off again back in England. Which could happen.
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