The Serie A powerhouse have appointed former Italian national team boss Spalletti as their manager for the rest of the term.
The 66-year-old steps in for Igor Tudor, who was dismissed following the loss to Lazio on Sunday.
Tudor arrived at the club in March filling the shoes of Thiago Motta but departed after seven months following a string of eight matches without a victory.
Spalletti guided Napoli to the Serie A crown in 2022-23 prior to assuming the Italy role, which missed out on the 2022 World Cup during Roberto Mancini's tenure.
Under his guidance, they made it to Euro 2024 exiting in the last 16 stage against Switzerland.
However, Spalletti was sacked by Italy 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'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 placing them seventh in the Serie A standings - six points adrift of Napoli at the top after nine rounds.
Their Champions League campaign sees them in 25th place in the group stage, after recording one defeat and two ties in the initial three fixtures.
