Barcelona announced the €40million signing of Arthur, the Gremio midfielder agreeing an immediate move to Camp Nou on a six-year deal.
Gremio had hoped to keep the 21 year-old for another season in the Brasileirao, having reached an agreement with the LaLiga champion in December 2017.
With the departures of Paulinho and Andres Iniesta combined with an injury to boom academy product Carles Alena, Barcelona has agreed to pay the €30 million fee plus €9 million add-ons to secure Arthur's services.
Arthur has been likened to Iniesta and Bayern's Thiago Alcantara - an an ex-Barca youth product - having famously completed 40 passes at a 100 per cent success rate during his Copa Libertadores debut last year.
He will relish the opportunity to link up with countryman Philippe Coutinho, as Barca's midfield rebuild continues.