This study examined the association between body composition, somatotype and socioeconomic status (SES) in Chilean children and adolescents by sex and school level (grade). The cross-sectional study was conducted on 1168 schoolchildren aged 6–18 years (572 males) from Valparaíso, Chile. Body composition, as assessed by percentage body fat (BF%) and somatotype, was evaluated using Ellis equations and the Heath–Carter method, respectively. The socioeconomic status of respondents was assessed using the ESOMAR survey. Obesity was defined as BF% ≥25 for boys and ≥30 for girls; ‘high endomorph’ somatotype was defined as a somatotype endomorph component (EC) of at least 5.5. Logistic regression analysis was used to assess the relationship between high adiposity and SES, potential confounding factors and school level. In females, the results indicated that the groups with lower SES had higher EC. At the 1st (youngest) school level (1–4th grades), males exhibited similar trends in their BF% and EC. High adiposity was associated with the female sex (BF%: OR=3.39; 95% CI 2.60, 4.41; high EC: OR=2.31; 95% CI 1.80, 2.98). In addition, low SES increased the risk of high adiposity compared with high SES (BF%: OR=2.25; 95% CI 1.40, 3.61; high EC: OR=2.19; 95% CI 1.37, 3.47). An association was observed between increased adiposity and lower SES, mainly in females, which indicates that females with low SES might be at greater risk of obesity.