Exercise tolerance is impaired in hypoxia, and it has recently been shown that dietary nitrate supplementation can reduce the oxygen (O₂) cost of muscle contractions. Therefore, we investigated the effect of dietary nitrate supplementation on arterial, muscle, and cerebral oxygenation status, symptoms of acute mountain sickness (AMS), and exercise tolerance at simulated 5,000 m altitude. Fifteen young, healthy volunteers participated in three experimental sessions according to a crossover study design. From 6 days prior to each session, subjects received either beetroot (BR) juice delivering 0.07 mmol nitrate/kg body wt/day or a control drink (CON). One session was in normoxia with CON (NOR_CON); the two other sessions were in hypoxia (11% O₂), with either CON (HYP_CON) or BR (HYP_BR). Subjects first cycled for 20 min at 45% of peak O₂consumption (VO₂peak; EX_45%) and thereafter, performed a maximal incremental exercise test (EX_max). Whole-body VO₂, arterial O₂saturation (%SpO₂) via pulsoximetry, and tissue oxygenation index of both muscle (TOI_M) and cerebral (TOI_C) tissue by near-infrared spectroscopy were measured. Hypoxia per se substantially reduced VO₂peak, %SpO₂, TOI_M, and TOI_C(NOR_CONvs. HYP_CON, P < 0.05). Compared with HYP_CON, VO₂at rest and during EX_45%was lower in HYP_BR( P < 0.05), whereas %SpO₂was higher ( P < 0.05). TOI_Mwas ∼4-5% higher in HYP_BRthan in HYP_CONboth at rest and during EX_45%and EX_max( P < 0.05). TOI_Cas well as the incidence of AMS symptoms were similar between HYP_CONand HYP_BRat any time. Hypoxia reduced time to exhaustion in EX_maxby 36% ( P < 0.05), but this ergolytic effect was partly negated by BR (+5%, P < 0.05). Short-term dietary nitrate supplementation improves arterial and muscle oxygenation status but not cerebral oxygenation status during exercise in severe hypoxia. This is associated with improved exercise tolerance against the background of a similar incidence of AMS.