The Old Lady have chosen ex- Italian national team boss Spalletti to lead the team for the remainder of the campaign.
The 66-year-old steps in for the Croatian manager, who was sacked after the Serie A side's defeat by Lazio on Sunday.
The former boss came on board in March as Thiago Motta's replacement but exited following a seven-month stint after an eight-game winless streak.
Spalletti led Napoli to the Serie A title in 2022-23 prior to assuming the Italy role, who had failed to qualify for the 2022 World Cup during Roberto Mancini's tenure.
Under his guidance, they made it to Euro 2024 where they were knocked out in the last 16 by 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 took interim charge of Juventus during the 3-1 victory over Udinese on Wednesday which left them seventh in Serie A - trailing leaders Napoli by six points after nine matches.
Their Champions League campaign sees them in 25th place in the group stage, with one loss and two draws in their first three games.
