This study aimed to identify major dietary patterns among Korean adolescents, examine their changes over the past 16 years, and evaluate differences by sex. Data were analyzed from 7,679 adolescents aged 12–18 years who participated in the 2007–2022 Korea National Health and Nutrition Examination Survey and completed health, examination, and nutrition surveys. Dietary intake was assessed using a 24-hour recall, and cluster analysis was performed based on the energy contribution of 26 food groups. Associations between dietary patterns and nutrient intake were examined using survey-weighted linear regression. Three dietary patterns were identified: Bread, Meat, & Dairy (33.1%); Rice-based Diet (45.5%); and Convenient Foods (21.3%). Among boys, the Rice-based Diet group showed the highest prevalence of obesity and the largest proportion of low-income households, whereas the Bread, Meat, & Dairy group exhibited higher rates of supplement use and high-income status (all p < 0.05). Among girls, the Convenient Foods group tended to be older and was more likely to skip breakfast, consume alcohol, eat out daily, and perceive themselves as overweight (all p < 0.05). Over the 16-year period, adherence to the Bread, Meat, & Dairy pattern increased, whereas adherence to the Rice-based Diet pattern declined in both sexes (p < 0.01). These findings highlight a shift toward Westernized dietary patterns among Korean adolescents. Accordingly, nutrition policies and interventions should adopt tailored strategies that account for both sex and socioeconomic differences to promote healthier eating habits and support long-term adolescent health.