The midfielder will join the Primera Liga leaders in the summer after deal struck.
Barcelona agree deal to sign Gremio's Arthur
Barcelona have reached agreement with Brazilian club Gremio to sign Arthur, a 21-year old midfielder.
The Spanish club said in a statement on Sunday that they can activate an option to sign Arthur in July by paying a basic €30 million (Dh135,600) plus €9m in add ons.
Earlier Spanish media reports had suggested Arthur would join next winter, after the South American campaign. The deal follows a sustained pursuit of the player by Barcelona.
Arthur helped propel Gremio to the final of the Copa Libertadores last year and was impressive as they beat Lanus of Argentina, 3-1, on aggregate in the final.
He did not finish the second leg, limping off in the second half with a torn ankle ligament. The injury meant Arthur missed the Club World Cup in December in Abu Dhabi where Gremio were beaten in the final by Real Madrid.
He will join compatriots Coutinho and Paulinho in the Barcelona midfield.
Meanwhile, Barcelona head coach Ernesto Valverde accepted his players had Chelsea and their Uefa Champions League tie on Wednesday on their minds after coasting through the second half of the 2-0 win over bottom club Malaga on Saturday night.
Goals from Luis Suarez and Philippe Coutinho set up Barca for their triumph against a side who played with 10 men for the final hour of proceedings after Samuel Garcia saw red for a foul on Jordi Alba.
Barcelona now turn their attentions to the second leg of their last-16 tie with Chelsea at Camp Nou, with the game delicately poised at 1-1 after the first leg in London in February.
"In the second half we began to subconsciously think about the match on Wednesday," Valverde told reporters.
"Scoring early here was crucial, the red card made the match easier for us.
"We will have to suffer to progress past Chelsea."
Ousmane Dembele continued to impress on his return from injury, setting up Coutinho's goal.
Valverde said: "Dembele always tries, he has a place here, but he is a player who needs time, is young and had injuries, also there is pressure on him.
"Today is yet another step forward for him."