Sonic the Hedgehog in Super Mario Bros. is a ROM hack, meaning it's an unauthorized modification of the original Super Mario Bros. game for the Nintendo Entertainment System (NES). It replaces Mario with Sonic and features elements inspired by the Sonic the Hedgehog series, such as loops and springs.  Gameplay retains the core platforming mechanics of Super Mario Bros. but incorporates new level design and elements influenced by the Sonic games.  This is a fan-made creation, not an official product from Sega or Nintendo.

Sonic the Hedgehog in Super Mario Bros.

Sonic 3D Blast 5 (Unl)