OBJECTIVE: To investigate associations of preterm birth with adiposity and body composition in adolescents and assess whether these associations differed by sex. METHODS: This study included 2171 adolescents aged 18-19 years from the São Luís population-based birth cohort, Brazil. Preterm birth (<37 weeks) was ascertained prospectively from the last menstrual period or, retrospectively, from maternal reports verified against the Brazilian Live Birth Information System. Outcomes were body mass index (BMI), waist circumference (WC), percentage fat mass (%FM), fat mass index (FMI), and fat-free mass index (FFMI). Linear regression models included preterm birth, sex, and their interaction and were adjusted for adolescent skin color and maternal smoking during pregnancy. A complete-case sensitivity analysis used the full DAG-informed adjustment set. RESULTS: In the primary adjusted models, preterm birth was associated among males with higher WC (β=3.59 cm; 95% CI: 1.03-6.15), %FM (β=3.38 %age points; 95% CI: 1.22-5.54), and FMI (β=1.10 kg/m²; 95% CI: 0.34-1.86), but not with BMI or FFMI. No associations were observed among females. Interactions between preterm birth and sex were significant for WC (P = 0.040), %FM (P = 0.008), and FMI (P = 0.015). In the sensitivity analysis, only %FM remained associated among males, and no interaction was statistically significant. CONCLUSION: Preterm birth was associated with greater adiposity among male adolescents in the primary analysis, but evidence was weaker after full adjustment. These findings suggest possible sex-related heterogeneity that requires confirmation in studies with more complete prenatal data.