The Old Lady have chosen former Italian national team manager Spalletti to take the helm for the remainder of the campaign.
The experienced coach replaces Igor Tudor, who was let go post the Lazio setback on Sunday.
Tudor had joined the club in March filling the shoes of Thiago Motta but exited following a seven-month stint 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, which missed out on the 2022 World Cup during Roberto Mancini's tenure.
He secured their spot in Euro 2024 but were eliminated in the round of 16 by Switzerland.
Yet, the Italian FA let Spalletti go in June post a qualifying defeat against Norway, as Italy faced the possibility of not making a third World Cup in a row.
Spalletti's managerial career includes Inter Milan, Roma, and Udinese in Serie A and secured two Russian Premier League titles with Zenit St Petersburg.
Juventus were temporarily led by Massimo Brambilla for their 3-1 win against Udinese on Wednesday placing them seventh in the Serie A standings - six points behind leaders Napoli after nine games.
They are 25th in the league phase of the Champions League, after recording one defeat and two ties in the initial three fixtures.
