Which Pokémon game introduced the Johto region?

Pokémon Gold and Silver introduced the Johto region.

Released for the Game Boy Color, Gold and Silver are the main games of Pokémon’s second generation. Their story begins in Johto, a region located west of Kanto, and follows a new Pokémon Trainer through eight Gym challenges and a confrontation with Team Rocket.

Johto added 100 new Pokémon, including Chikorita, Cyndaquil, Totodile, Lugia, Ho-Oh, and Celebi. The games also introduced breeding, held items, a day-and-night clock, and the Dark and Steel types.

After completing the Johto League, players can travel to Kanto, the setting of the first-generation games. This post-game journey was a major feature because it allowed players to revisit familiar locations with new challenges. Pokémon Crystal later expanded the same Johto setting with additional story and presentation changes.