Background <p>This study assesses associations between eating behavior and physical activity (PA) and overweight/obesity in children and adolescents, focusing on differences depending on familial socio-economic status (SES).</p> Methods <p>Data were collected within a school-based study. The sample comprised 661 8- to 15-year-old children and adolescents from families with either low (<i>n</i> = 77), medium (<i>n</i> = 367), or high (<i>n</i> = 217) SES. Overweight, including obesity, was defined as a Body Mass Index (BMI) standard deviation score above the 90th percentile. Eating behavior was assessed using the parent-version of the Composition and Culture of Eating Questionnaire (CoCu). Regarding PA, we compared children performing versus not performing any PA in their leisure time. Logistic regression analyses were applied to assess associations between overweight and healthiness of diet, culture of eating (media use while eating and snacking between meals), and leisure PA. All associations were checked for interactions with SES.</p> Results <p>The prevalence of overweight was 22% in low SES families, 14% in medium SES families, and 6% in high SES families. Overweight was significantly associated with a less healthy diet, but this association was only shown in children from families with medium SES. Media use while eating and snacking between meals were more frequent in children with overweight, while PA was less frequent. Family SES did not moderate the strengths of these associations.</p> Conclusions <p>Unhealthier diet and eating habits as well as less PA are associated with overweight in children and adolescents. However, associations with healthy diets were not observed in families with low or high SES, suggesting that other factors may play a greater role in these groups.</p>