Barcelona have confirmed that Arthur is joining Juventus.
They say the deal for the 23-year old is worth €72million plus and additional €10million in potential add-ons.
It's expected an announcement regarding Miralem Pjanic's move in the other direction will come later on Monday.
The Brazilian Melo will stay with Barcelona until their season is completed.
Arthur joined Barca from Gremio for €30million in the summer of 2018, and has made 72 appearances for the club.
He's scored four times in 28 appearances so far this season, with his most recent appearance a four-minute cameo in the weekend's 2-2 draw with Celta Vigo in La Liga.
The potential swap deal has puzzled some, with Pjanic having just turned 30 in April.
Spanish football journalist Dermot Corrigan tweeted on Sunday night, "Since [Andoni] Zubizarreta was fired as sporting director in Jan 2015 - Barca have spent more than €800 million on new players, without even one of them establishing themselves as a senior team leader."
In that time, there have been big money arrivals like Philippe Coutinho, Ousmane Dembele, Malcom and Antoine Griezmann who - through a mixture of form and fitness - haven't made lasting impressions at the Camp Nou.