The agreement allows Barca to sign Arthur, the Brazilian midfielder, in July 2018 for a fee of £26.7m plus around £8m in add-ons.
FC Barcelona issued an official statement after midday on Sunday to announce that they have reached an agreement with Grêmio de Porto Alegre to reserve an option to buy Brazilian midfielder Arthur Henrique Ramos de Oliveira Melo.
And Barca have now confirmed that they have secured the option to complete the signing of the Brazil youth global in July.
Arthur has represented Brazil at Under-17 and Under-20 level and helped Gremio win the 2017 Copa Libertadores.
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LaLiga giants FC Barcelona have confirmed the signing of a Brazilian star in a deal reportedly worth €30 million.
The club's technical director Robert Fernandez travelled to Brazil a few weeks ago to finalize the transfer of the 21-year-old who is expected to arrive in Cataluña at the season.
Barcelona do not, as it stands, have a place available for Arthur with Yerry Mina, Philippe Coutinho and Paulinho taking up the three slots.
Messi will return when Barca host Chelsea in a Champions League last-16 second leg game on Wednesday but captain Andres Iniesta is fighting to return from a hamstring injury.