The Spain Under-20 starlet, who bagged 18 goals in the Segunda Division last season, becomes new Toffees boss Roberto Martinez's fourth signing of the summer transfer window.
The 19-year-old attacker, who was a part of Spain's squad for the Under-20 World Cup in Turkey, scored 18 times in 33 matches for the Catalan club's 'B' team last season.
"Gerard is a real diamond of Spanish football and a boy with incredible footballing talent and I am delighted that he has chosen to take the next step of his career with Everton," Everton boss Roberto Martinez told the club's official website.
Deulofeu made one appearance in La Liga last season, coming off the bench against Mallorca, and also featured in Champions League group stage ties against Spartak Moscow and Benfica.
However, the Masia academy product's game time at Camp Nou was expected to be limited next season following Barcelona's capture of Brazilian superstar Neymar.
The Blaugrana loanee is Everton's fourth signing of the summer transfer window, with the Toffees having already secured permanent deals for ex-Wigan trio Arouna Kone, Joel Robles and Antolin Alcaraz.