Which mainline Pokémon games were the first to use fully 3D gameplay?

Pokémon X and Y were the first mainline Pokémon games to use fully 3D gameplay.

Nintendo and Game Freak released Pokémon X and Y worldwide on October 12, 2013, for the Nintendo 3DS. The games shifted the core series from the primarily two-dimensional presentation used by earlier handheld entries to 3D environments and character models.

The games introduced the Kalos region, inspired by France, and added Fairy-type Pokémon, Mega Evolution, character customization, and Pokémon-Amie. They also supported online features through the Player Search System, making interaction with other players more visible during ordinary play.

Pokémon Diamond and Pearl had already used some three-dimensional elements, especially in environments, so “first 3D Pokémon games” can be an imprecise description. X and Y are specifically recognized for bringing fully 3D presentation to the mainline series. Later games expanded the visual approach further.