Fati, 17, burst onto the scene last season, scoring eight goals in all competitions, but remained registered to the B team, despite never actually playing for them.
His performances earned him a new contract in December until 2022, with his release clause initially rising from €100m to €170m.
However, the terms of that deal stipulated that once he was officially given first team status, the clause would more than double to €400m.
Fati will inherit the No.22 shirt which was vacated this week by Arturo Vidal, who joined Inter Milan. He grew particularly close to the Chilean midfielder during his debut season at Camp Nou.
He became Barca's youngest ever scorer in La Liga when he scored a header against Osasuna aged 16 years and 304 days last August.
Three months later, a goal against Inter Milan at the San Siro made him the youngest player to ever score in the Champions League, aged 17 years and 40 days.
Those feats earned him an international call up and he became the Spanish national team's youngest ever scorer when he netted against Ukraine earlier this month.