The Old Lady have appointed ex- Azzurri manager Spalletti as their manager until the end of the season.
The experienced coach steps in for Igor Tudor, who was sacked after the Serie A side's defeat by Lazio last weekend.
Tudor arrived at the club in March filling the shoes of Thiago Motta but left after seven months following a string of eight matches without a victory.
Under Spalletti, Napoli clinched the Serie A championship in 2022-23 then moving on to lead the Italian national team, which missed out on the 2022 World Cup under former boss Roberto Mancini.
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 after a World Cup qualifying defeat by 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 secured two Russian Premier League titles with 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 - 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.
