Background: Whether body composition is associated with lung function in asthmatic children has not been investigated. This study aimed to primarily investigate whether BMI z-score and body composition were associated with respiratory function in asthmatic children. Methods: In a cross-sectional study, male (n = 27; mean age: 11.9 y (SD: 2.3)) and female (n = 21; mean age: 13.6 y (SD: 2.2)) asthmatic children underwent clinical assessment. Results: BMI z-score was associated with forced expiratory volume in 1 s (FEV1; r = 0.458), forced vital capacity (FVC; r = 0.477), and total lung capacity (TLC; r = 0.451) in males only (P < 0.05). Total lean mass was associated with FEV1 (r = 0.655), FVC (r = 0.562), and TLC (r = 0.635) in males, as was thoracic lean mass (FEV1 (r = 0.573), FVC (r = 0.526), and TLC (r = 0.497); P < 0.05). TLC was associated with total (r = 0.522) and thoracic (r = 0.532) lean mass in females (P < 0.05). Fat mass was not associated with lung function in this group. Conclusion: Lean mass, not fat mass, is associated with lung function in children with asthma. The positive association between BMI z-score and respiratory function in male children is driven by lean mass. Although body weight can be easily monitored in the clinical setting, body composition can provide important information. Future research exploring lean mass and lung function associations could inform future interventions.