The Old Lady have appointed ex- Italian national team head coach the veteran manager to lead the team for the rest of the term.
The 66-year-old replaces his predecessor, who was let go after the Serie A side's defeat by Lazio recently.
Tudor had joined the club 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 before taking charge of Italy, after the team's failure to reach the 2022 World Cup under former boss Roberto Mancini.
He helped them qualify for Euro 2024 where they were knocked out in the last 16 by Switzerland.
However, Spalletti was sacked by Italy 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 won the Russian Premier League twice 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 behind leaders Napoli after nine games.
In the Champions League, they sit 25th in the league phase, having lost once and drawn twice in their opening three matches.
