The Bavarian powerhouse said on Tuesday Kim signed a five-year deal through June 2028 and will wear the No. 3.
"He impresses with his physical presence, as well as his mentality and speed," Bayern CEO Jan-Christian Dreesen said. "We're pleased he can immediately take part fully in preseason and firmly believe that he will also excite our fans with his style of play."
"We are convinced that he is the right person to strengthen the team in the future, together with Thomas Tuchel's coaching team and technical director Marco Neppe," Dreesen said.
Bayern had been looking to bolster its back line following Lucas Hernández's departure for Paris Saint-Germain and with Benjamin Pavard's future uncertain. Pavard's contract with Bayern expires next year and he reportedly wants to leave the club.
Kim was honored as Serie A's best defender last season after helping Napoli to their first league title in more than three decades in his debut season. He played all but three of Napoli's 38 league games, chipping in two goals. Kim joined from Turkish club Fenerbahçe the previous summer.
It has been a swift ascent for the towering 26-year-old defender, who played for K-League teams Gyeongju KHNP and Jeonbuk Hyundai Motors before switching to Chinese Super League team Beijing Guoan in 2019. He joined Fenerbahçe three seasons later, spending just one season at the Turkish club before joining Napoli for a reported fee of around €18m ($20m).
The club's sporting director Salihamidžić was famously fired after Bayern clinched an 11th consecutive Bundesliga title on the final day of the season. Bayern were second going into the final round, where it beat Borussia Dortmund to the title thanks to a late goal from Jamal Musiala coupled with Dortmund's draw with Mainz.
Salihamidžić was Bayern's sporting director since 2017. The club also fired former goalkeeper Oliver Kahn as CEO. Kahn was replaced by Dreesen.
Information from The Associated Press was used in this story.