The Old Lady have named ex- Italian national team manager the veteran manager to take the helm until the end of the season.
The 66-year-old takes over from Igor Tudor, who was dismissed after the Serie A side's defeat by Lazio recently.
Tudor arrived at the club in March to replace Thiago Motta but departed after seven months after an eight-game winless streak.
Spalletti guided Napoli to the Serie A crown in 2022-23 before taking charge of Italy, which missed out on the 2022 World Cup with Mancini at the helm.
He secured their spot in Euro 2024 exiting in the last 16 stage against Switzerland.
However, Spalletti was sacked by Italy in June following a loss to Norway in World Cup qualifying, as Italy faced the possibility of not making a third World Cup in a row.
He has previously coached Inter Milan, Roma, and Udinese in Italy's top flight and secured two Russian Premier League titles with Zenit St Petersburg.
Juventus were temporarily led by Massimo Brambilla in the midweek 3-1 triumph over Udinese moving them to seventh position in the league - six points adrift of Napoli at the top after nine rounds.
Their Champions League campaign sees them in 25th place in the group stage, having lost once and drawn twice in their opening three matches.
