ONE Championship is the largest MMA promotion in Asia and the second-largest in the world, but its ruleset is meaningfully different from the UFC's. The differences are not cosmetic — they change how fighters train, how they cut weight and how fights look on broadcast.
Open weight class structure with hydration tests
The single biggest rules difference is how ONE handles weight. The UFC, like virtually every Unified Rules promotion, weighs fighters once on the day before the fight, then allows them roughly 24 hours of rehydration and refeeding before they compete. The standard practice is severe dehydration to make the contracted weight, followed by 10-30 pounds of rebound water and glycogen weight before fight time.
ONE Championship abolished day-before weigh-ins and replaced them with a weekly weight and hydration testing protocol during fight week. Fighters must stay within a defined weight band and meet a specific urine specific gravity (a hydration measurement) repeatedly during fight week and on fight day morning. Anyone who tests dehydrated, or who is outside their weight band on test day, is removed from the contract or moved up in weight. Fighters also compete in a higher weight class than the UFC equivalent: a UFC lightweight (155 lb cutdown) typically competes at ONE featherweight (155 lb walking-around weight). The intent is that fighters compete close to their natural, hydrated walking weight rather than at an artificial cut. This was introduced after the 2015 death of a ONE fighter from complications of extreme weight cutting.
Global Martial Arts ruleset
ONE's MMA ruleset is broadly close to the Unified Rules — same fouls list for most categories, same 10-point must scoring, same round structure (3 rounds of 5 minutes, 5 for title fights) — with two significant differences. Soccer kicks (kicking a downed opponent in the head from a standing position) are legal in ONE. Knees to the head of a downed opponent are also legal. Both of these are banned under the Unified Rules. ONE allows them because they are part of the broader Asian and historical MMA traditions (PRIDE-era Japanese MMA permitted both), and ONE's medical staff has taken the position that, with appropriate referee oversight, they are acceptable.
Elbows on the ground remain legal as under the Unified Rules. The 12-to-6 elbow is also legal in ONE, which has never adopted the New Jersey-origin prohibition. Small-glove striking (4 oz MMA gloves) is the same as elsewhere.
Separate Muay Thai, kickboxing, and submission grappling cards
ONE is unusual among major promotions in running separate full Muay Thai cards (with the small 4 oz gloves rather than traditional 8 oz boxing gloves), full kickboxing cards (similar to GLORY's K-1 derived ruleset), and submission grappling cards (essentially no-gi BJJ matches with their own ruleset). A single event might mix all four disciplines on one card. Each ruleset is judged on its own terms — Muay Thai bouts are scored in a Western 3-round 10-point system rather than the traditional Thai cumulative system.
Eastern fighting tradition influence
ONE's ruleset openly references PRIDE-era Japanese MMA, traditional Muay Thai, Lethwei, sanda and other Asian martial arts. This is the source of soccer kicks, downward knees and the overall presentation. ONE markets itself explicitly as a global martial arts promotion rather than a North American sports league.
How ONE addresses dehydration in a way UFC doesn't
The hydration testing protocol is the most important rules-adjacent difference. Under UFC rules, a fighter's contract is determined by what they can make at the morning weigh-in; how they got there is not regulated. Under ONE rules, fighters who attempt extreme cuts simply fail the hydration test and are scratched. The downstream effects include fighters competing closer to their natural size, less catastrophic fight-week dehydration, fewer collapses and hospitalisations during weight cuts, and weight classes that reflect actual walking-around size rather than depleted weigh-in size. The trade-off is that fighters who have built careers around large cuts must redesign their entire approach to fight prep.
How this changes weight-cutting strategy
A UFC fighter typically targets walking-around-weight roughly 15-25 lb above their division ceiling, manipulates water and sodium in the final 5 days, cuts dramatically in the final 24-36 hours, then rehydrates and refeeds. A ONE fighter cannot do this. They must train at or near their division ceiling year-round, manage food and water consistently throughout fight week, and pass repeated hydration checks. The strategic effect is that ONE fighters are more often competing against opponents of genuinely similar size and conditioning, with less of the post-cut weakness and rebound that shapes the first round of many UFC fights. Some UFC fighters who have moved to ONE have moved up a weight class as a result.