Deulofeu had a successful loan season at Everton in 2013-14 but spent much of the last campaign on the sidelines during a spell at Sevilla.
The 21-year-old has signed a three-year deal and will move on 1 July.
Everton boss Roberto Martinez believes Deulofeu can still develop into one of the game's outstanding young talents.
He told Everton's website: "We are getting an even more mature player now than he was a couple of seasons ago and a player who considers Everton as home.
"This is why we are incredibly excited to have him back here on a permanent basis and to encourage him to grow, develop and mature his enormous talent."
Martinez forged a close relationship with the player during their season together and Deulofeu believes his countryman can revive his career.
He has turned down other more lucrative offers to return to Merseyside.
Personal terms have been agreed and it is believed Barcelona will insert a buy-back option into the deal for some future point should he develop as they once hoped.
Deulofeu will become Everton's second signing of the summer after England midfield man Tom Cleverley joined on a free transfer from Manchester United following a successful loan spell at Aston Villa.
Martinez said he was "really pleased" to complete the signings early in the transfer window.
He added: "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."

The pair has dispensation to compete at London 2012 as host nation players, but have qualified by right for Rio.
Drinkhall, 26, and Pitchford, 22, helped England's men win bronze at this year's World Team Championships.
"We are delighted that Team GB have two qualified singles players for the first time in 20 years," said GB's table tennis team leader Simon Mills.
Drinkhall and Pitchford, who are ranked 60th and 51st in the world respectively, won a combined five medals at the 2014 Commonwealth Games.
Everton have re-signed Barcelona forward Gerard Deulofeu on a permanent basis after agreeing a £4.3m deal for the one-cap Spain international.
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Liam Pitchford and Paul Drinkhall have been selected to represent Great Britain at this summer's Rio Olympics.