Which Pokémon game was the first to feature the Johto region?

Pokémon Gold and Silver were the first games to feature the Johto region.

The paired Game Boy Color titles introduced Johto as the setting for the second generation of main-series Pokémon games. Players begin in New Bark Town, explore Johto's routes and cities, challenge its eight Gym Leaders, and eventually face the Pokémon League.

Johto is closely connected to Kanto. After completing the main Johto storyline, players can travel to Kanto, revisit locations from the first games, and challenge another set of Gyms. This post-game journey helped make Gold and Silver feel much larger than their predecessors.

Pokémon Crystal later expanded the same generation with an updated story, animated battle sprites, and a more prominent role for Suicune. Johto is sometimes mistakenly identified as the setting of the first games because Kanto and Johto appear together in Gold, Silver, and Crystal.