Mental Health and Physical Activity among Adolescents from Different Social Backgrounds: Results of the German COMO-Study Summary: Objective: Investigating the effects of the COVID-19 pandemic is central to future health strategies. This article provides population-based data on the mental health and physical activity of children and adolescents in Germany, considering sociodemographic characteristics. Method: The data refer to the population-based representative COMO-study with 5240 participants aged 4 to 17 years, surveyed between October 2023 and February 2024. It assessed mental health problems (SDQ), health-related quality of life (HRQoL, KIDSCREEN-10); and physical activity (WHO physical activity recommendation, sport) and calculated prevalence stratified by age, gender, and socioeconomic status (SES) as well as regression analyses. Results: 21 % of youth reported mental health problems, 14.5 % had low HRQoL, and 19.8 % fulfilled the WHO activity recommendation. Adolescents with low SES showed more mental health problems (29.9 % low vs. 10.7 % high), lower HRQoL, and were less active both in sports clubs (46.8 % low vs. 77.1 % high) and outside sports clubs. Physical activity was associated with fewer mental health problems (OR = 0.66-0.74) and higher HRQoL (OR = 1.19-1.73). Differences in age and gender showed heterogeneous results. Conclusions: Five years into the pandemic, socioeconomic inequalities in mental health, well-being, and physical activity persist. Physical activity can be a promising complementary approach to promoting mental health in youth.