What genre are the main Sonic the Hedgehog games?

The main Sonic the Hedgehog games are platformers.

Platform games challenge players to guide a character through stages filled with obstacles, enemies, jumps, platforms, and hazards. Sonic’s version emphasizes speed, momentum, slopes, springs, loops, and collectible rings. The first game, released in 1991 for the Sega Genesis, established this format and helped make Sonic Sega’s mascot and a major rival to Nintendo’s Mario.

The main series includes both two-dimensional and three-dimensional platformers. Early titles such as Sonic the Hedgehog, Sonic 2, Sonic 3, and Sonic & Knuckles use side-scrolling gameplay. Sonic Adventure, released for the Dreamcast in 1998, was the first major mainline Sonic game built around 3D gameplay.

The franchise also contains racing, fighting, party, sports, role-playing, and visual-novel spin-offs. Those genres can make the broader series classification seem less precise, but the principal Sonic games are conventionally identified as platformers.