Psychological immunity is important for the athlete to handle stress, manage emotions, and have resilience in competitive sports. This study examines a correlation between volleyball players’ psychological immunity and their sense of conscience, to define the impact of mental resilience on the conscientiousness, ethical decision-making and self-discipline of athletes. Eighty-four volleyball players from seven clubs in Iraq were included in this study whose information was gathered by validated psychological immunity and vitality of conscience questionnaires. Correlation analysis, ANOVA, and regression modelling were used to evaluate the strength and significance of this relationship. These findings showed significant positive correlation between psychological immunity and conscientiousness, of which self-regulation had the highest correlation (r = 0.874r = 0.874). The remaining 3.8% was attributed to individual differences on the conscientiousness measure, which qualified the effect of psychological immunity on athletes’ behaviour, based on ANOVA results. The regression analysis also showed a positive predictive relationship, suggesting that reinforcing psychological immunity can increase ethical accountability and the stability of performance. Such results have implications for the inclusion of mental resilience training as part of athlete development programmes to promote performance excellence and ethical sportsmanship. Towards this end, future research is needed to investigate the longitudinal effects of various psychological training interventions as well as their use across different sports disciplines to expand our understanding of how the development of psychological immunity ultimately translates into athletic performance and moral decision-making.