Which Pokémon game was the first to introduce the Hoenn region?

Pokémon Ruby and Sapphire were the first Pokémon games to introduce the Hoenn region.

Released for the Game Boy Advance in Japan on November 21, 2002, Ruby and Sapphire began Pokémon's third generation. Their story takes place in Hoenn, a region inspired partly by Japan's Kyushu and neighboring areas. The games introduced 135 new Pokémon, bringing the series' total species count to 386 at that point.

Ruby and Sapphire also added abilities, natures, double battles, Pokémon Contests, and two competing villain teams: Team Magma and Team Aqua. These systems expanded battles and gave the games a distinct identity beyond the earlier Kanto and Johto adventures.

The pair later received an enhanced version, Pokémon Emerald, and remakes called Pokémon Omega Ruby and Alpha Sapphire. Those later titles revisit Hoenn but do not precede Ruby and Sapphire as the region's debut.