Which Pokémon game was the first to use a full 3D battle arena?

Pokémon Stadium was the first Pokémon game to use a full 3D battle arena.

Released for the Nintendo 64 in Japan in 1998, Pokémon Stadium presented battles with fully polygonal Pokémon models, animated attacks, stadium crowds, and multiple tournament cups. It was developed by Nintendo EAD and supported battles involving Pokémon from the original Game Boy games.

The game was not a conventional role-playing adventure. Instead, its main focus was competitive battling in stadium events, along with mini-games and other battle-related features. A Transfer Pak allowed compatible Pokémon from Pokémon Red, Green, Blue, and Yellow to be used on the Nintendo 64.

Pokémon Colosseum and Pokémon Battle Revolution are sometimes named as the first 3D Pokémon battle games because they offered more advanced visuals and story modes. They were released later, however. Pokémon Stadium established the franchise’s recognizable 3D battle presentation before those console titles.