The Serie A powerhouse have named ex- Italy boss the veteran manager to lead the team for the rest of the term.
The experienced coach replaces Igor Tudor, who was dismissed after the Serie A side's defeat by Lazio recently.
The former boss came on board in March to replace Thiago Motta but exited following a seven-month stint at the end of an eight-match winless run.
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 helped them qualify for Euro 2024 exiting in the last 16 stage against Switzerland.
Yet, the Italian FA let Spalletti go in June post a qualifying defeat against Norway, putting Italy in danger of missing a third straight World Cup.
Spalletti's managerial career includes Inter Milan, Roma, and Udinese in Serie A and lifted the Russian league trophy twice while at Zenit St Petersburg.
Massimo Brambilla served as interim manager for Juventus for their 3-1 win against Udinese on Wednesday placing them seventh in the Serie A standings - six points adrift of Napoli at the top after nine rounds.
In the Champions League, they sit 25th in the league phase, after recording one defeat and two ties in the initial three fixtures.
