What is the name of the first 3D Sonic platformer?

The first fully 3D Sonic platformer was Sonic Adventure. Released for the Sega Dreamcast in Japan in 1998, it moved the series from side-scrolling stages into expansive 3D environments built around speed, exploration, and cinematic storytelling.

Sonic Adventure followed several earlier experiments with three-dimensional presentation. Sonic 3D Blast used an isometric viewpoint, Sonic Jam included a small 3D exploration area, and Sonic X-treme was planned as a fully 3D Saturn game but was cancelled. These projects are why the wording can sometimes cause confusion.

Sonic Adventure introduced six playable characters, each with different abilities and story routes. Sonic focused on fast platforming, while Tails could fly, Knuckles could glide and dig, Amy used a hammer, Big fished, and E-102 Gamma used ranged attacks. Their stories converged around the water creature Chaos and the Chaos Emeralds.

The game was also important because it established movement and combat ideas that became standard in later 3D Sonic games, including the Homing Attack and Light Speed Dash. Sonic Adventure 2 expanded this formula, but it was the original Sonic Adventure that made the series' major transition to 3D.