Environmental literacy (EL) equips individuals to understand the interactions between natural and human systems and to act sustainably. Despite its importance, research suggests secondary school students in Nigeria exhibit limited environmental awareness, impairing local sustainability efforts. This study adopts a quantitative ex post facto design to examine the collective and individual influence of the Big Five personality traits (openness, conscientiousness, extraversion, agreeableness, and neuroticism) on environmental literacy among secondary school students in Ondo State, Nigeria. A stratified random sample of 462 students completed validated questionnaires and critical thinking assessments. Multiple regression analysis revealed that the Big Five traits significantly predicted environmental literacy jointly (Adj. R² = 0.217, F(5,456)=26.52, p<.001). Conscientiousness emerged as the strongest positive predictor, followed by agreeableness and extraversion, while openness and neuroticism had no significant independent effect. Findings highlight the value of incorporating personality-focused strategies into environmental education. Recommendations include targeted workshops for teachers and students and further research into personality-led engagement approaches.