Purpose: The main purpose of the present study was to investigate the association between sex and body mass index (BMI) as mediated by the temperament traits postulated by the regulative theory of temperament. Methods: A group of 317 subjects, including 122 men and 195 women 18-82 years old (M = 31.93; SD = 12.64 years), was recruited from the general population to participate in the study. A cross-sectional design was applied in this study. Temperament was assessed using the formal characteristics of behaviour-temperament inventory revised version (FCB-TIR). In the sample, BMIs ranged from 16.51 to 35.56 kg/m2 (M = 23.31 kg/m2; SD = 3.61 kg/m2). Results: The male group had significantly higher BMI, briskness and endurance levels and significantly lower perseveration, sensory sensitivity and emotional reactivity levels compared to the female group. No significant differences between these groups were noted in terms of activity and rhythmicity. The correlations between BMI and briskness and between BMI and endurance were positive, while the correlations between BMI and perseveration and between BMI and activity were negative. The correlations among BMI, sensory sensitivity, emotional reactivity and rhythmicity were not statistically significant. The mediation analysis showed that briskness and endurance were significant partial mediators of the relationship between sex and BMI. Conclusions: Sex and two temperament traits, briskness and endurance, were the best predictors of BMI. Briskness and endurance partially mediated the relationship between sex and BMI. Level of evidence: Descriptive cross-sectional study. Level V.