The Serie A powerhouse have appointed ex- Italy head coach the veteran manager to lead the team for the rest of the term.
Spalletti steps in for his predecessor, who was let go after the Serie A side's defeat by Lazio 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.
Under Spalletti, Napoli clinched the Serie A championship in 2022-23 then moving on to lead the Italian national team, after the team's failure to reach 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.
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's managerial career includes Inter Milan, Roma, and Udinese in Serie A and won the Russian Premier League twice with Zenit St Petersburg.
Massimo Brambilla took interim charge of Juventus for their 3-1 win against Udinese on Wednesday moving them to seventh position in the league - six points behind leaders Napoli after nine games.
In the Champions League, they sit 25th in the league phase, with one loss and two draws in their first three games.
