Jurgen Klopp bolstered his midfield options after Jordan Henderson suffered a hamstring injury in Wednesday’s Premier League win against Newcastle.
Henderson is the latest Liverpool midfielder to be hit by the injury bug, with Thiago Alcantara, Fabinho, Naby Keita and Alex Oxlade-Chamberlain also dealing with fitness issues.
Arthur joined Juventus from Barcelona in 2020 and made 31 appearances for the Serie A club last season.
The 26-year-old also helped Brazil win the Copa America title in 2019.
“I’m really happy to be here wearing this great shirt with this famous badge that represents so much in world football. It’s a dream,” Arthur said.
“We talked a lot, and our ideas and visions were a good fit so I’m sure it was the right choice. I’m really happy and highly motivated to continue living my dream on the pitch and giving my all in a Liverpool shirt.
“Liverpool have a wonderful history in football. I’ve played against Liverpool, I know what it feels like to be in the stadium with the fans behind them.
“I was with the opposition and now I’m on the right side so I’m really excited. It’s a massive club with a great manager, great players.”