The Serie A powerhouse have appointed former Azzurri boss Spalletti to take the helm for the remainder of the campaign.
The experienced coach steps in for Igor Tudor, who was let go post the Lazio setback on Sunday.
Tudor had joined the club in March as Thiago Motta's replacement but departed after seven months after an eight-game winless streak.
Under Spalletti, Napoli clinched the Serie A championship in 2022-23 before taking charge of Italy, 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.
Nonetheless, Italy dismissed Spalletti 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 in the midweek 3-1 triumph over Udinese placing them seventh in the Serie A standings - trailing leaders Napoli by six points after nine matches.
They are 25th in the league phase of the Champions League, with one loss and two draws in their first three games.
