The 19-year-old - regarded as one of the brightest stars in the four-time European champions' academy - scored 18 times in 32 appearances for Bareclona's B side last season and recently signed a long-term contract.
Manager Roberto Martínez told the Everton website: "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."
His arrival marks Martinez' fourth signing in three days after he brought in former Wigan players Arouna Kone and Antolin Alcaraz and Atletico Madrid goalkeeper Joel Robles, who was also at the DW Stadium last season.
Deulofeu is a graduate of Barcelona's famed La Masia and has played for Spain's Under-17s, Under-19s, Under-20s and Under-21s.
He will join up with the Blues squad later this summer as he is currently on a break after involvement in the recent Under-20s World Cup in Turkey.
Most of the squad are now in Austria preparing for a friendly against Austria Vienna on Saturday.
Captain Phil Jagielka has welcomed the new arrivals, particularly the experience of Kone and Alcaraz.
"It is nice to have those type of players on our team this time," said the England centre-back.
"Obviously they are two players that the manager knows very well, two players who were big stars at Wigan.
"I know Kone played most of their games and scored most of their goals and was a really big handful.
"Alcaraz unfortunately had a few injuries last season and didn't play as much football as he would have liked but, the same again, he was a very important player for Wigan.
"They are two fantastic signatures to get and hopefully they can come into what is now a good squad and make it a little bit stronger."
Jagielka added on evertontv: "We've only got four centre halves including Shane (Duffy) so we haven't got that many.
"The manager played different formations at Wigan, if he's going to play three at the back here, four isn't a lot of centre-halves to choose from - especially if a couple get injured.
"It's definitely important that we've got strength in numbers and he's a fantastic player and I'm sure he'll be competing for a starting role."
In his first season as Everton captain, Jagielka is keen to ensure the new arrivals feel welcome and settle in quickly.
"I think on the whole in the changing room it is easy to come in and within a week you feel like you've been here a season," he added.
"I'll do the usual things, introducing, and if they need anything, obviously, they can come and see me.
"But I think the lads themselves will welcome the new signings into the club with open arms and I'm pretty sure they'll find it easy to adjust."