The Serie A powerhouse have appointed former Azzurri head coach the veteran manager as their manager for the rest of the term.
The experienced coach takes over from his predecessor, who was dismissed post the Lazio setback last weekend.
The former boss came on board in March to replace Thiago Motta but exited following a seven-month stint at the end of an eight-match winless run.
Under Spalletti, Napoli clinched the Serie A championship in 2022-23 prior to assuming the Italy role, which missed out on the 2022 World Cup under former boss Roberto Mancini.
He secured their spot in Euro 2024 where they were knocked out in the last 16 by Switzerland.
However, Spalletti was sacked by Italy in June post a qualifying defeat against Norway, with the Azzurri at risk of missing out on a third consecutive 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.
Juventus were temporarily led by Massimo Brambilla for their 3-1 win against 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, having lost once and drawn twice in their opening three matches.
