Sonic Gaiden is a 2003 fan-made hack of the original Sonic the Hedgehog game for the Sega Genesis/Mega Drive.

The hack is a co-operative effort between two noteworthy hackers in the community, Yuski the Dog and Esrael Neto. Esrael is most famous for his work on Sonic 2 Delta, a hack of the Sonic 2 Beta ROM which re-coded all the incomplete elements and restored deleted levels. Yuski had created a previous hack in 2000 called Mighty the Armadillo which was a sprite and palette hack of the U.S. Sonic 1 ROM. Sonic Gaiden was born when Esrael e-mailed Yuski a Sonic 1 ROM-hack with both Sonic and Mighty playable.