Anxiety incurs debilitating psychological and physiological stress which inhibits both physical and cognitive performance and can be manifested in two forms; state and trait. State anxiety is short-lived and reflects a transitionary emotional state, whereas trait anxiety is a stable characteristic reflecting an anxious predisposition. Extreme environmental conditions such as heat stress and hypoxia also induce physiological and psychological stress decreasing physical and cognitive performance. However, large inter-individual variability in response to exercise in the heat and in hypoxia have been reported. This study aimed to investigate the impact of trait anxiety independently, and in combination with state anxiety, on physical performance in the heat and in hypoxia. Following ethical approval from Loughborough University, 28 healthy males (mean ± SD; 22.1 ± 2.9 years) were recruited, in which 14 were allocated to the heat stress group (35°C, 50% relative humidity (RH)) and 14 to the hypoxic stress group (0.010 FiO2, 21°C, 50%RH). Participants in each group completed one familiarisation session and two experimental trials. For the heat group this included: 1) heat stress with no state anxiety and 2) heat stress with state anxiety; and for the hypoxic group this included: 1) normobaric hypoxia with no state anxiety and 2) normobaric hypoxia with state anxiety. State anxiety was induced using a mental maths protocol with accompanying environmental deception. Each condition included cycling on a bike ergometer at 70% VO2max until voluntary exhaustion (EXH). Objective measures included oxygen consumption and heart rate, and subjective measures included state anxiousness and rate of perceived exertion. Trait anxiety was determined using the state-trait anxiety inventory. Results indicated a significant decrease in EXH between the state anxiety and non-state anxiety conditions in both heat (-276s, p = 0.03) and hypoxia (-367, p = 0.002). A significant negative correlation was observed in the heat where the higher trait anxious participants performed worse (r2= 0.48, p = 0.006). The correlation was stronger when state anxiety was induced (r2= 0.67, p < 0.001). In hypoxia however, the relationship with trait anxiety independently (r2= 0.07, p = 0.354) and combined with state anxiety (r2= 0.17, p = 0.142) was weak. The results from this study indicate that independently, trait anxiety inhibits performance in heat stress but not hypoxia. Further, state anxiety reduces performance in both heat and hypoxic conditions. Therefore, it is critical that interindividual trait psychological characteristics such as anxiety are considered in understanding interindividual variability in performance research. The development of psychophysiological coping mechanisms to mitigate the impact of trait and state anxiety are warranted, particularly in extreme environments.