The Old Lady have appointed ex- Azzurri boss Spalletti to take the helm for the remainder of the campaign.
The experienced coach takes over from Igor Tudor, who was dismissed post the Lazio setback last weekend.
Tudor arrived at the club in March filling the shoes of Thiago Motta but left after seven months after an eight-game winless streak.
Spalletti guided Napoli to the Serie A crown 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.
However, Spalletti was sacked by Italy in June following a loss to Norway in World Cup qualifying, as Italy faced the possibility of not making a third World Cup in a row.
Spalletti has also managed 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 in the midweek 3-1 triumph over Udinese moving them to seventh position in the league - six points adrift of Napoli at the top after nine rounds.
They are 25th in the league phase of the Champions League, after recording one defeat and two ties in the initial three fixtures.
