The teenager's existing deal was set to expire at the end of the 2022-23 season.
Gavi has become a key player for the club after breaking through in August 2021, shortly before being given his Spain debut by Luis Enrique in October 2021, becoming his country's youngest ever player at senior level.
Gavi scored against the Czech Republic in June 2022 to become Spain's youngest ever goalscorer, taking the record from his club team-mate Ansu Fati.
Both players, along with Pedri, Ronald Araujo and Ferran Torres have the same one billion euro release clause written into their Barcelona deals, the joint most expensive in world football.
Talks over Gavi's renewal had dragged on but Barcelona wanted to wait until he turned 18 to finalise the deal, so it could be longer than a three-year duration, while the club also had to work to clear space on their books due to La Liga's stringent internal financial fair play rules.
Gavi has 54 appearances for Barcelona and 10 caps for Spain so far.
