On a final evening of frenzied negotiations, Juve announced Liverpool have an option to buy Arthur for $37.29 million. The 26-year-old has played 63 times for the Turin club since his move from Barcelona in 2020.
"We talked a lot and our ideas and visions were a good fit so I'm sure it was the right choice," Arthur said in a statement.
"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. It's a dream to be here."
Arthur, who can play in different roles in central midfield, arrives at Liverpool who are reeling from an injury crisis with midfielders Thiago Alcantara, Jordan Henderson, Naby Keita and Alex Oxlade-Chamberlain all sidelined.
Elsewhere, the usual last-minute activity between agents and clubs went to the wire - though there was no move for Manchester United's Cristiano Ronaldo.
Chelsea rounded out their lavish spending spree this summer by bringing Pierre-Emerick Aubameyang back to the Premier League and taking Juve midfielder Denis Sakaria on a season-long loan.
Former Arsenal striker Aubameyang, 33, only joined Barcelona in January but has fallen behind Robert Lewandowski in the pecking order.
Likewise Zakaria, 25, a Switzerland international, signed for Juve in January from Borussia Monchengladbach.
Gabon striker Aubameyang says he has "unfinished business" in the Premier League.
Leicester City moved quickly to replace Wesley Fofana after the Frenchman was sold to Chelsea for £70 million by signing Belgian defender Wout Faes from Ligue 1 side Reims, their first major acquisition of the summer.
Fulham signed Paris Saint-Germain left-back Layvin Kurzawa on a season-long loan and took Wales winger Daniel James on loan from Leeds United.
The Cottagers also brought Willian back to the Premier League after the Brazilian winger left Corinthians in August.
Willian, formerly of Chelsea and Arsenal, agreed a one-year contract, saying: "I'm happy to be back in the Premier League. Fulham is a club that wants to improve, that wants to fight for something bigger."
200 of the biggest summer transfers
Senegal midfielder Idrissa Gana Gueye returned to Everton from PSG for a nominal fee on a two-year contract.
The 32-year-old played for Everton from 2016 to 2019 before moving to PSG.
Brighton & Hove Albiion signed Scotland midfielder Billy Gilmour from Chelsea for £7.5 million.
Wolves let Leander Dendoncker leave, with the Belgian midfielder joining Aston Villa for £13 million.
Southampton signed Manchester City's teenagers Samuel Edozie and Juan Larios for a combined £16 million, landed Croatia centre-back Duje Caleta-Car from Marseille and took Ainsley Maitland-Niles on loan from Arsenal.
Against the background of Britain's cost of living crisis, Premier League clubs showed no caution, reportedly breaking the all-time record spending total for an entire season with an incredible £2 billion changing hands in just one transfer window.