The Serie A powerhouse have chosen previous Italy manager Spalletti to lead the team for the remainder of the campaign.
The 66-year-old takes over from his predecessor, who was sacked 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 exited following a seven-month stint following a string of eight matches without a victory.
Spalletti led Napoli to the Serie A title in 2022-23 prior to assuming the Italy role, which missed out on the 2022 World Cup with Mancini at the helm.
He helped them qualify for Euro 2024 but were eliminated in the round of 16 by Switzerland.
Nonetheless, Italy dismissed Spalletti in June after a World Cup qualifying defeat by Norway, with the Azzurri at risk of missing out on a third consecutive World Cup.
He has previously coached Inter Milan, Roma, and Udinese in Italy's top flight and won the Russian Premier League twice with Zenit St Petersburg.
Massimo Brambilla took interim charge of Juventus in the midweek 3-1 triumph over Udinese which left them seventh in Serie A - six points adrift of Napoli at the top after nine rounds.
They are 25th in the league phase of the Champions League, after recording one defeat and two ties in the initial three fixtures.
