The Mysterious Class (Korean: 남고괴담; RR: Namgogoedam; lit. "Ghost Story in a Boys' School") is a 2021 South Korean web series featuring an ensemble cast consisting of the members of boy band Treasure—Choi Hyun-suk, Park Ji-hoon, Yoshinori Kanemoto, Kim Jun-kyu, Yoon Jae-hyuk, Asahi Hamada, Kim Do-young, Haruto Watanabe, Park Jeong-woo, So Jung-hwan, Mashiho Takata, and Bang Ye-dam. Its plot centers on a group of 12th grade students in a boys' school who try to solve the mystery of their class' resident ghost.

The series aired for free viewing every Friday 22:00 KST on the band's official YouTube channel from 12 November to 31 December 2021. Short clips containing post-credits scenes for each episode were also uploaded for paid subscription in the band's Weverse account a day after its corresponding episode's airing date. It served as Treasure's first full-fledge endeavor in acting after the band made its acting debut through the short film It's Okay, That's Friendship that was broadcast on 5 March 2021 as the 36th episode of their variety web series Treasure Map.