Dietary nitrate supplementation enhances sea level performance and may ameliorate hypoxemia at high altitude. However, nitrate may exacerbate acute mountain sickness (AMS), specifically headache. This study investigated the effect of nitrate supplementation on AMS symptoms and exercise responses with 6h hypoxia. Twenty recreationally-active men (mean(SD): age 22(4) years, V ̇ O2max 51(6) mL·min-1 ·kg-1) completed this randomized double-blinded placebo-controlled crossover study. Twelve participants were classified as AMS-based on Environmental Symptom Questionnaire (AMS-C) score <0.7 in both trials, and five participants were classified as AMS+ based on AMS-C score ≥0.7 on placebo. Five days nitrate supplementation (70mL beetroot juice containing ~6.4mmol nitrate daily) increased plasma NO metabolites by 182µM compared to placebo but did not reduce AMS or improve exercise performance. After 4h hypoxia (FIO2=0.124) nitrate increased AMS-C and headache severity (visual analogue scale (VAS); whole sample ∆10[1,20] mm; p=0.03) compared to placebo. In addition, after 5h hypoxia, nitrate increased sense of effort during submaximal exercise (∆7[-1,14]; p=0.07). In AMS-nitrate did not alter headache or sense of effort. In contrast, in AMS+ nitrate increased headache severity (∆26[-3,56] mm; p=0.07), sense of effort (∆14[1,28]; p=0.04), oxygen consumption, ventilation, and mean arterial pressure during submaximal exercise. On the next day, in a separate acute hypoxic exercise test (FIO2=0.141), nitrate did not improve time to exhaustion at 80% hypoxic V ̇ O2max. In conclusion, dietary nitrate increases AMS and sense of effort during exercise, particularly in those who experienced AMS. Dietary nitrate is therefore not recommended as an AMS prophylactic or ergogenic aid non-acclimatized individuals at altitude