Background <p>Rates of overweight and obesity in children are increasing progressively worldwide, which can negatively impact chest mechanics and lung function. However, children at different stages of growth may be impacted differently by obesity, highlighting the need to assess maturation status related to body growth, as the maturity offset.</p> Objective <p>To investigate the effect of body mass index (BMI) on respiratory muscle strength (RMS) and lung function in children, considering maturity offset status as a covariate.</p> Methods <p>This cross-sectional analytical study included 90 children aged 5–14 from two Brazilian public schools. BMI was calculated and converted into percentiles for age and sex to classify children as overweight/obese. RMS, including maximal inspiratory pressure (MIP) and maximal expiratory pressure (MEP), was measured through a digital manometer, and spirometry was used to assess lung function. The age-to-peak height velocity (APHV) formula was used to determine the maturity offset status. Data were compared between groups, and linear regression was used to examine the effect of BMI on RMS and spirometric variables, adjusted for sex, age, and maturity offset status.</p> Results <p>MIP (Δ= +14.12 cmH₂O, <i>p</i> = 0.015) was higher in overweight/obese children. Regression models indicated a relationship between %MEP (β: 0.142; CI 95%: -1.163; 1.453) and BMI, considering age, sex, and maturity offset status as covariates.</p> Conclusion <p>Overweight/obesity was associated with higher inspiratory muscle strength and related to %MEP. This relationship was influenced by the covariates of age, sex, and maturity compensation status, indicating that anthropometric variables need to be considered in future studies.</p>