Sonic 1 Co-op is a ROM hack of Sonic the Hedgehog for the Sega Genesis that introduces cooperative gameplay. Unlike the original single-player experience, this hack allows two or more players to experience the game simultaneously.  This requires significant modification to the game's mechanics, requiring adjustments to level design and potentially the introduction of new features to enable cooperative play. The exact implementation of co-op functionality would vary.

Sonic 1 Co-op

Sonic 1 CDSS Edition