Juventus have chosen previous Italian national team manager Spalletti as their manager until the end of the season.
The 66-year-old takes over from Igor Tudor, who was let go after the Serie A side's defeat by Lazio last weekend.
The former boss came on board in March as Thiago Motta's replacement but departed after seven months after an eight-game winless streak.
Under Spalletti, Napoli clinched the Serie A championship in 2022-23 before taking charge of Italy, after the team's failure to reach the 2022 World Cup during Roberto Mancini's tenure.
He helped them qualify for Euro 2024 where they were knocked out in the last 16 by Switzerland.
Nonetheless, Italy dismissed Spalletti in June post a qualifying defeat against Norway, as Italy faced the possibility of not making a third World Cup in a row.
Spalletti's managerial career includes Inter Milan, Roma, and Udinese in Serie A and won the Russian Premier League twice with Zenit St Petersburg.
Massimo Brambilla served as interim manager for Juventus during the 3-1 victory over Udinese on Wednesday which left them seventh in Serie A - trailing leaders Napoli by six points after nine matches.
In the Champions League, they sit 25th in the league phase, having lost once and drawn twice in their opening three matches.
