The Serie A powerhouse have appointed ex- Italy manager the veteran manager to lead the team for the remainder of the campaign.
Spalletti replaces Igor Tudor, who was sacked post the Lazio setback on Sunday.
The former boss came on board 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 then moving on to lead the Italian national team, who had failed to qualify for 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.
However, Spalletti was sacked by Italy in June following a loss to Norway in World Cup qualifying, putting Italy in danger of missing a third straight World Cup.
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.
Massimo Brambilla served as interim manager for Juventus during the 3-1 victory over Udinese on Wednesday moving them to seventh position in the league - 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.
