Which Pokémon game was the first core-series title to use the Hoenn region?

Pokémon Ruby and Sapphire were the first core-series games to use the Hoenn region.

Game Freak developed the pair for the Game Boy Advance, and Nintendo released them in Japan in 2002 before international launches in 2003. Hoenn is a region shaped by water and volcanic terrain, with the cities, routes, and Pokémon encounters providing a new setting separate from the earlier Kanto and Johto adventures.

Ruby and Sapphire introduced the third generation of Pokémon games. They added 135 new species, bringing the series total to 386 at the time, and introduced abilities, natures, double battles, Pokémon Contests, and the player’s choice between Team Magma and Team Aqua as major antagonistic forces.

The games also began a change in compatibility: Pokémon from the original Game Boy titles could not be transferred directly into Ruby and Sapphire. Later remakes, Pokémon Omega Ruby and Alpha Sapphire, returned to Hoenn on the Nintendo 3DS, but those are updated versions rather than the region’s first appearance.