The Old Lady have chosen previous Italian national team boss the veteran manager to lead the team for the remainder of the campaign.
The 66-year-old takes over from the Croatian manager, who was let go following the loss to Lazio on Sunday.
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.
Under Spalletti, Napoli clinched the Serie A championship in 2022-23 before taking charge of Italy, who had failed to qualify for the 2022 World Cup during Roberto Mancini's tenure.
Under his guidance, they made it to Euro 2024 but were eliminated in the round of 16 by Switzerland.
Nonetheless, Italy dismissed Spalletti 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 lifted the Russian league trophy twice while at 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.
They are 25th in the league phase of the Champions League, with one loss and two draws in their first three games.
