The Catalans added that Fati will now sport the No. 22 shirt last worn by Arturo Vidal prior to the Chilean's move to Inter Milan on Tuesday.
Fati, who has been training with Barcelona's first team since last summer, made 33 appearances in all competitions for the club last season.
It's been a whirlwind year for the Guinea-Bissau-born winger capped by Wednesday's official announcement. Fati became Barcelona's youngest-ever goalscorer and the youngest player to record a goal in the Champions League as a 16-year-old last season.
Fati then earned a full international Spain debut on Sept. 3 against Germany and became La Roja's youngest-ever scorer three days later versus Ukraine.
Barcelona start their 2020-21 La Liga campaign at Camp Nou against Villarreal on Sept. 27.