Excessive body weight during adolescence represents a significant public health problem worldwide. Identifying factors associated with its development is crucial. We estimated the prevalence of overweight and obesity in a representative sample of 11, 13 and, 15-year-olds living in Greece and explored the association with diet-related behaviours and habits. Self-reported data on weight, height, diet-related behaviours and habits were used from 3816 students (1898 boys, 1918 girls) participants in the Greek arm of the international Health Behaviour in School-Aged Children (HBSC) study during 2018. Overweight and obesity were defined using the 2007 WHO growth charts classification. Prevalence of overweight was 19.4% in the total sample, 24.1% for boys and 14.7% for girls, and prevalence of obesity was 5.3% in the total sample, 7.3% for boys and 3.4% for girls, respectively. In the total sample, overweight (including obesity) was positively associated with male gender, low family affluence, skipping breakfast, and being on a diet, and inversely associated with age and being physically active. Eating rarely with the family was positively associated with overweight only among boys and eating snacks/meals in front of screens only among girls. No association was noted for eating in fast-food restaurants, consuming vegetables, fruits, sweets, and sugar-sweetened beverages.

Background/Objectives: Healthy diet, regular physical activity (PA), and minimizing sedentary behavior (SB) are crucial in promoting adolescents’ health and well-being. We assessed adherence to PA and SB recommendations among a representative sample of adolescents and explored their relationship with diet and diet-related behaviors. Methods: Data from the Greek arm of the 2017/2018 international Health Behavior in School-Aged Children (HBSC) cross-sectional study were used, including a probability sample of 3357 students (47.6% boys) who were 11, 13, and 15 years old. PA, SB, consumption of food groups/beverages, and diet-related behaviors were self-reported. Multiple logistic regression was used to identify potential associations. Results: Most students failed to meet PA (83.9%) (i.e., ≥1 h of moderate-to-vigorous physical activity/day) and SB (90.2%) (i.e., ≤2 h of screen time/day) recommendations. Daily consumption of fruit and vegetables compared to consumption at ≤1 day/week was associated with increased adherence to PA recommendations (adjusted Odds Ratio (aOR) = 2.26, 95% Confidence Intervals (CI): 1.62–3.17 for fruit; aOR = 1.35, 95%CI: 1.00–1.82 for vegetables). Eating sweets ≤1 day/week vs. every day was associated with higher adherence to SB recommendations (aOR = 2.41, 95%CI: 1.43–4.04). Poor diet quality was related to lower adherence to PA and SB recommendations. Daily breakfast consumption vs. never and abstaining from eating at fast-food restaurants were associated with better adherence to PA and SB recommendations. Rarely eating in front of screens was associated with substantially higher odds of adhering to SB recommendations (aOR = 5.79, 95%CI: 3.67–9.14). Conclusions: Healthier diet/diet-related behaviors were associated with increased PA and reduced SB in this sample of adolescents.