The following are known Game Genie Codes for Sonic the Hedgehog on SEGA Game Gear.

Note: There are two versions of the game, if the first set does not work, try the second set.
Version 0
• Infinite Lives
  007-01A-3BE
• Rings Worth 2 Points
  020-54A-E6E
• Rings Worth 3 Points
  030-54A-E6E
• Rings Worth 4 Points
  040-54A-E6E
• Rings Worth 5 Points
  050-54A-E6E
• Rings Worth 6 Points
  060-54A-E6E
• Rings Worth 7 Points
  070-54A-E6E
• Rings Worth 8 Points
  080-54A-E6E
• Never Lose Rings
  3A0-21C-2A2
• Stop Timer
  003-F8C-3B7
• Start With 1 Life
  013-7DE-E66
• Start With 5 Lives
  053-7DE-E66
• Start With 9 Lives
  093-7DE-E66

Version 1

• Start On Green Hill Zone – Part 2
  003-8CE-5D4
  013-88E-91A
• Start On Green Hill Zone – Part 3
  003-8CE-5D4
  023-88E-91A
• Start On Bridge – Part 1
  003-8CE-5D4
  033-88E-91A
• Start On Bridge – Part 2
  003-8CE-5D4
  043-88E-91A
• Start On Bridge – Part 3
  003-8CE-5D4
  053-88E-91A
• Start On Jungle – Part 1
  003-8CE-5D4
  063-88E-91A
• Start On Jungle – Part 2
  003-8CE-5D4
  073-88E-91A
• Start On Jungle – Part 3
  003-8CE-5D4
  083-88E-91A
• Start On Labyrinth – Part 1
  003-8CE-5D4
  093-88E-91A
• Start On Labyrinth – Part 2
  003-8CE-5D4
  0A3-88E-91A
• Start On Labyrinth – Part 3
  003-8CE-5D4
  0B3-88E-91A
• Start On Scrap Brain – Part 1
  003-8CE-5D4
  0C3-88E-91A
• Start On Scrap Brain – Part 2
  003-8CE-5D4
  0D3-88E-91A
• Start On Scrap Brain – Part 3
  003-8CE-5D4
  0E3-88E-91A
• Start On Sky Base – Part 1
  003-8CE-5D4
  0F3-88E-91A
• Don’t Lose Breath Underwater
  2A2-86A-6E2