The following are known Game Genie Codes for Greatest Heavyweights on SEGA Genesis.

• BJ6A-ACD6
  Rounds are 1 minute
• BJ6A-AED6
  Rounds are 2 minutes
• BJ6A-AJD6
  Rounds are 4 minutes
• BJ6A-AND6
  Rounds are 6 minutes
• BJ6A-AWD6
  Rounds are 9 minutes
• CTXT-AA46
  Rounds are infinitely long so you must win by knockout
• AJ4T-AA2A
  Infinite rounds and you must win by knockout
• RH7A-A6YA
  Set attributes to whatever you want when creating a new boxer
• RH9A-A6TE
  Attributes are not reduced after a fight
• RH9T-A6VN + BM9T-AA9W
  Pick as many training items as you want