Sodium supplements are ubiquitous in endurance running. Their impact on hydration, dysnatremia, and running pace, all independently associated with performance, has been the subject of much debate. The objective of the study was to assess the effect of sodium supplementation normalized to body weight, as a predictor of race performance in ultramarathon runners. Prospective observational study during an 80 km (50 mi) stage of a 6-stage 250 km (155 mi) ultramarathon in Chile, Patagonia, Namibia and Mongolia. Finish line hydration status, serum sodium, and questionnaires were gathered to define sodium ingestion categories at 33% and 66% both for weight-adjusted and true total sodium consumption, and analyzed for significant relationships to dysnatremia, hydration, and impact on race performance. Results:266 participants were enrolled, with 218 (82%) with complete sodium supplement rate data, with 174 (80%) with finish line sodium, and 161 (74%) with both pre-race weights and total sodium ingestion allowing for normalization of both total amount (mg/hr) and weight-based (mg/hr/kg) rate of ingestion analysis. Participants were stratified as low (<200 mg/hr; <2.79 mg/hr/kg), medium (200 to 360 mg/hr; 2.79 mg/hr/kg - 4.78 mg/hr/kg), or high (>360 mg/hr; >4.78 mg/hr/kg) sodium intake categories. Sodium intake did not impact the pace (0.99), total race time (p=1), quintile rank (p=0.4), incidence of dysnatremia (p=0.74), or quantity of hydration (p=0.7). These outcomes did not change using binary grouping around the 50% ingestion amount, assessed by temperature of race, or when excluding the 34 (27%) dysnatremic runners. When hydration status was controlled by weight-adjusted normalization of sodium intake, dehydration was found to improve performance with faster times [n = 40 (28.1% ), pace = 9.4 min / km (± 2.1), time = 12.4 hrs (±2.9)] compared to euhydration [n = 62 (43.8%), pace = 10.6 min / km (± 2.8), time = 13.8 hrs (±3.7)] and overhydration [n=40 (28.1%), pace = 13 min / km (± 3.4), time = 17 hrs]. Sodium supplements in ultramarathon runners did not meaningfully impact ultramarathon performance. Those runners who were most dehydrated were found to have improved performance.