The Serie A powerhouse have named former Italy head coach the veteran manager as their manager until the end of the season.
The experienced coach takes over from the Croatian manager, who was dismissed following the loss to Lazio recently.
The former boss came on board in March filling the shoes of Thiago Motta but left after seven months at the end of an eight-match winless run.
Spalletti guided Napoli to the Serie A crown in 2022-23 then moving on to lead the Italian national team, after the team's failure to reach the 2022 World Cup with Mancini at the helm.
He helped them qualify for Euro 2024 where they were knocked out in the last 16 by Switzerland.
Yet, the Italian FA let Spalletti go in June following a loss to Norway in World Cup qualifying, with the Azzurri at risk of missing out on a third consecutive World Cup.
Spalletti has also managed Inter Milan, Roma and Udinese in Serie A and secured two Russian Premier League titles with Zenit St Petersburg.
Massimo Brambilla served as interim manager for Juventus during the 3-1 victory over Udinese on Wednesday placing them seventh in the Serie A standings - trailing leaders Napoli by six points after nine matches.
In the Champions League, they sit 25th in the league phase, after recording one defeat and two ties in the initial three fixtures.
