Pokémon Red, Green, and Blue take place in the Kanto region.
Kanto is the setting for the first-generation Pokémon games and contains locations such as Pallet Town, Viridian City, Cerulean City, and the Pokémon League. Players travel across the region while earning Gym Badges and building a team of Pokémon.
The name Kanto was not used in the original Japanese games themselves. It was introduced with the second generation, when Pokémon Gold and Silver added the Johto region and allowed players to revisit the first game's land. Retrospectively, the original setting is universally known as Kanto.
Kanto also became the setting for the opening arc of the Pokémon anime. That connection helped make locations such as Pallet Town and Viridian City familiar to viewers who encountered Pokémon through television rather than the Game Boy games.