Gavi also signed a new contract with a release clause of 1 billion Euros which has not been registered until now.
The 18-year-old has been playing as a player of Barca Atletic but with the departure of Memphis Depay, Hector Bellerin and the retirement of Gerard Pique, Barca have been judged to have space in their salary cap.
A Spanish court rules in support of Barca as La Liga declined their previous request to register Gavi. They celebrated this with a post on the club's social media page of Gavi wearing the No. 6 jersey.
Gavi will wear the jersey on Wednesday as they face Real Betis in La Liga.