The Old Lady have named previous Azzurri boss Spalletti to take the helm for the remainder of the campaign.
The 66-year-old takes over from Igor Tudor, who was dismissed post the Lazio setback last weekend.
Tudor had joined the club in March filling the shoes of Thiago Motta but departed after seven months following a string of eight matches without a victory.
Spalletti guided Napoli to the Serie A crown in 2022-23 prior to assuming the Italy role, who had failed to qualify for the 2022 World Cup under former boss Roberto Mancini.
He helped them qualify for Euro 2024 but were eliminated in the round of 16 by Switzerland.
Nonetheless, Italy dismissed Spalletti 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 lifted the Russian league trophy twice while at Zenit St Petersburg.
Juventus were temporarily led by Massimo Brambilla during the 3-1 victory over Udinese on Wednesday 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, with one loss and two draws in their first three games.
