Body dissatisfaction is prevalent among adolescents and linked to adverse outcomes. This study examined sociodemographic and anthropometric factors associated with body dissatisfaction in Spanish adolescents. Cross-sectional data from the Eating Healthy and Daily Life Activities (EHDLA) study (N = 1378 students aged 12–17 years) were analyzed. Body dissatisfaction was assessed using the Body Image Dimensional Assessment (BIDA) instrument (overall index and three subscales). Variables included age, sex, socioeconomic status, immigrant status, family structure, parental education, area of residence, and body mass index. Robust multivariable linear models adjusted for all a priori covariates were fitted. Multivariable complete-case models included n = 747 of 1378 adolescents. Younger age, female sex, and excess weight were associated with higher body dissatisfaction on one or more BIDA dimensions. Excess weight showed the largest standardized associations and was linked to all BIDA dimensions (p < 0.001). Maternal educational attainment was not significant in the fully adjusted primary models but remained associated with lower Body Dissatisfaction Score in a reduced-model sensitivity analysis. No significant associations were observed for socioeconomic status, immigrant status, family structure, or area of residence in the primary models. Younger adolescents, female adolescents, and those with excess weight represent subgroups at greater risk of body dissatisfaction. These findings underscore the need for targeted prevention and intervention strategies addressing both anthropometric and sociodemographic risk factors during adolescence.