Deulofeu has been linked with a return to Serie A, but is now back on Barca's books after Ernesto Valverde's club exercised their right to bring the winger back to Camp Nou.
"Barcelona have activated the buy-back clause for Gerard Deulofeu," read the club's statement on Friday.
"In the following days, the terms will be agreed with both Everton and the player. Deulofeu's contract will run until 30 June 2019."
Deulofeu played 17 times for Milan in Serie A after joining them on loan in January, scoring four times, and his strong performances earned a re-call to the senior Spain squad.
Prior to his permanent move to Everton, reportedly worth £4.2m, he had spent a year on loan with at Goodison Park in 2013-14 and spent the following season at Sevilla.
A Barcelona youth product, he has only made six first-team appearances for the club, two of them coming in LaLiga.