SUMMARY: Leptin is a hormone produced by adipose tissue with key roles in metabolism, immunity, and appetite regulation. Its expression has also been identified in lung tissue. While the relationship between leptin and respiratory function has been explored, particularly in the context of obesity, there is limited specific evidence regarding its association with functional residual capacity (FRC), its components, and gender-based differences. The aim of this study was to determine the relationship between FRC and plasma leptin levels, differentiated by sex, in a young adult population. This cross-sectional study included 64 healthy young adults (32 women and 32 men), in whom anthropometric variables, body composition by bioelectrical impedance, pulmonary parameters by body plethysmography, and plasma leptin levels by ELISA were assessed. Results showed that women had significantly higher leptin levels and body fat percentage, whereas men presented significantly higher values of FRC, residual volume (RV), and expiratory reserve volume (ERV). In the overall analysis, leptin levels were significantly and inversely correlated with RV (r=- 0.469; p=0.0001), ERV (r=-0.552; p=0.0001), and FRC (r=-0.633; p=0.0001). When stratified by sex, these associations remained significant only in men, with a moderate inverse correlation between leptin and FRC (r=-0.542; p=0.0001). No significant correlations were found in women. The findings support the conclusion that there is an inverse relationship between leptin levels and FRC in young adults, primarily in males. Therefore, it is recommended that leptin levels, airway resistance tests, and pulmonary volumes be evaluated and interpreted jointly to enhance understanding of respiratory function.