The 21-year-old joins the Catalan giants from Serie A outfit Gremio in a deal worth €31 million.
Arthur, who is Barca's first signing for next season, will sign a six-year contract with the LaLiga champions.
"FC Barcelona and Gremio de Porto Alegre have agreed the transfer of the player Arthur Henrique Ramos de Oliveira Melo, following the agreement the two clubs reached in March. The agreement will see the Brazilian player immediately join the squad," According to a statement on the Barca website.
"The cost of the transfer is off €31 million with €9 million in variables. The player will sign a contract with the Club for the following six seasons, until the end of the 2023/24 campaign; his buyout clause is to be set at €400 million.
"The schedule for the presentation of Arthur Henrique Ramos de Oliveira Melo as a new first team player will be announced shortly."