Prevalence of diet-related behaviors (i.e., breakfast consumption, eating with the family) and their association with a 17-point diet quality score, constructed on the basis of reported frequency (in days/week) of vegetable, fruit, sweets and sugar-sweetened beverages consumption, was investigated among 3525 adolescents (51.5% girls) aged 11, 13 and 15 years, who were participants in the Greek arm of the international Health Behaviour in School-Aged Children (HBSC) cross-sectional study, during 2018. Almost one-third (32.9%) of the sample had breakfast ≤1 day/weekdays, 20.2% rarely ate with the family, 26.1% had a meal while watching TV ≥5 days/week, 31.7% had a snack in front of a screen ≥5 days/week and 24.1% ate in fast-food restaurants at least once/week. Multivariable ordinal logistic regression revealed that eating breakfast ≤1 day/weekdays compared to 4-5 days/weekdays (Odds ratio (OR): 1.56, 95% con-fidence interval (CI): 1.34-1.82), eating rarely with the family compared to almost every day (OR: 1.35, 95% CI: 1.13-1.60) and eating in fast-food restaurants ≥2 times/week vs. rarely (OR: 4.59, 95% CI: 3.14-6.70) were associated with higher odds of having poor diet quality. High frequency of having meals/snacks in front of a screen/TV was also associated with poor diet quality. Efforts to prevent or modify these behaviors during adolescence may contribute to healthier diet.