Barcelona have announced the €40million signing of Arthur, the Gremio midfielder agreeing an immediate move to Camp Nou on a six-year deal.
Gremio hoped to retain Arthur until the end of the year, but Barca on Monday confirmed they had exercised the option they took out on the midfielder last March.
The Catalan giants will pay an initial €31m fee with as much as €9m due to be paid in variables as part of the transfer for the 21-year-old, whose release clause has been set at €400m.
It had been suggested Barca would agree to loan Arthur back to Gremio, but he will immediately link up with Ernesto Valverde's squad for the 2018-19 season.
Arthur committed to LaLiga's champions until 2024 and will take the place vacated by fellow Brazilian midfielder Paulinho, who has returned to Chinese Super League side Guangzhou Evergrande after a single season at Camp Nou.
He helped Gremio win the Copa do Brasil and the Copa Libertadores but will now have a chance to showcase his talent on the European stage.
Barca announced earlier that young trio Oscar Mingueza, Jandro Orellana and Guillem Jaime have agreed new contracts at the club.
La Masia has suffered a talent drain in recent years but Mingueza and Jandro have committed to the club until 2021, with their deals including the option of a two-year extension.
Guillem Jaime has signed with Barca until 2020, his terms also including the potential to renew for two more seasons. All three players helped the Catalan side win last season's UEFA Youth League.