The Serie A powerhouse have named previous Azzurri manager Spalletti to lead the team until the end of the season.
Spalletti replaces Igor Tudor, who was sacked after the Serie A side's defeat by Lazio on Sunday.
Tudor arrived at the club in March to replace Thiago Motta but exited following a seven-month stint after an eight-game winless streak.
Spalletti guided Napoli to the Serie A crown in 2022-23 prior to assuming the Italy role, after the team's failure to reach the 2022 World Cup with Mancini at the helm.
Under his guidance, they made it to Euro 2024 exiting in the last 16 stage against Switzerland.
Nonetheless, Italy dismissed Spalletti in June post a qualifying defeat against Norway, with the Azzurri at risk of missing out on a third consecutive World Cup.
Spalletti's managerial career includes Inter Milan, Roma, and Udinese in Serie A and won the Russian Premier League twice with Zenit St Petersburg.
Juventus were temporarily led by Massimo Brambilla for their 3-1 win against Udinese on Wednesday moving them to seventh position in the league - six points behind leaders Napoli after nine games.
They are 25th in the league phase of the Champions League, having lost once and drawn twice in their opening three matches.
