Quercetin is a naturally-occurring polyphenolic flavonol, commonly consumed in the human diet and associated with numerous health benefits. Quercetin supplementation has been shown to increase muscle oxidative capacity, endurance, and voluntary physical activity in mice, but its effect on physical performance and other important variables in moderately-trained humans is unknown. Using a randomized, double-blind, repeated-measures, placebo-controlled design, this research was conducted to determine the effects of chronic quercetin supplementation on physical performance, mood, sleep, and illness in 58 healthy, moderately-trained young men and women undergoing regular military physical training. In the first study, the effects of 6 weeks of quercetin supplementation on peak oxygen consumption (VO2peak) during maximal-effort uphill treadmill running, four physical performance tests [Army Physical Fitness Test (APFT), Baumgartner Modified Pull-Up Test (BMPU), Wingate Anaerobic Test (WAnT), and 36.6-m sprint], and a simple reaction time test [Walter Reed palm-held psychomotor vigilance test] were evaluated before and after supplementation with 1 g/d of quercetin with vitamins and other substances in a soft chew or a placebo chew. Pretreatment-to-posttreatment changes in VO2peak and physical performance were not significantly different (p > 0.05). Pre-, mid-, and posttreatment changes in simple reaction time were not significantly different between groups (p > 0.05). In the second study, the effects of 6 weeks of chronic quercetin supplementation on the transient moods of energy and fatigue, sleep, and illness were examined prior to, in the middle, at the end, and 2 weeks following supplementation with 1 g/d of quercetin with vitamins and other substances in a soft chew or a placebo chew. Changes in energy and fatigue, sleep quality, and self-reported illness rate and severity were not significantly different (p > 0.05) between groups. Outcome measures were not influenced by the sex of the participants. In conclusion, these results indicate that chronic dietary quercetin supplementation in moderately-trained young men and women conducting regular military physical training does not improve VO2peak, physical performance, or simple reaction time. Results also show that the transient moods of energy and fatigue, sleep quality, and illness rate and severity are unaffected by chronic quercetin supplementation in this population.