Osomatsu-kun: Hachamecha Gekijō (おそ松くん はちゃめちゃ劇場, "Osomatsu-kun: Nonsense Theater") is a platform video game by Sega, which was released in 1988 for the Sega Mega Drive.^[2][3] An early Mega Drive title, it never saw a release outside of Japan.^[3] The game was based on the manga Osomatsu-kun by Fujio Akatsuka, and acts as a promotion for the anime adaptation that premiered the same year; players controlled the title character and encountered many of the series' characters in a strange (but comical) world.^[2]

The player controls Osomatsu, leader of the sextuplets, through a total of three stages, armed only with a short-range slingshot. Each stage has a distinct path the players need to follow in order to proceed, though each stage has a variety of different places to go. Each stage is divided into a number of separate segments, with Osomatsu traversing between them via a variety of ways, such as going through doors, falling down pits, grabbing balloons, or jumping on a golden cloud. In order to fight the boss of each stage, the player has to locate and defeat the stage's miniboss, portrayed by either Chibita or Dekapan, in order to locate and defeat Iyami, who plays the role of the stage's boss. Every time a boss or miniboss is defeated, Osomatsu's maximum health and slingshot range both increase.

The player can enter shops run by Totoko, where they can purchase power-ups using collectible yellow ribbons; the player's inventory is located on the pause screen. The shops also have mini-games, such as a slot machine and a small path-finding game. The player starts each game with three lives and cannot earn any continues; additional lives are awarded by collecting six 1-Up Cards. Defeating enemies can cause them to randomly drop ribbons, health items, or (on rare occasions) 1-Up Cards.

The game was poorly received due to the maze-esque level design, frustrating controls and short length.^[4] Despite this, the game has obtained a cult following as an infamous kusoge.