The Old Lady have appointed previous Azzurri head coach the veteran manager to lead the team until the end of the season.
Spalletti replaces Igor Tudor, who was let go post the Lazio setback recently.
Tudor had joined the club in March as Thiago Motta's replacement 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 then moving on to lead the Italian national team, 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 following a loss to Norway in World Cup qualifying, as Italy faced the possibility of not making a third World Cup in a row.
He has previously coached Inter Milan, Roma, and Udinese in Italy's top flight and secured two Russian Premier League titles with Zenit St Petersburg.
Massimo Brambilla took interim charge of Juventus 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, having lost once and drawn twice in their opening three matches.
