This study provides the first national update on phase angle (PhA) as a biomarker of body composition, nutritional status, and health outcomes in Korean adults. Using data from 5,035 adults aged 20–80 years from the 2023 Korea National Health and Nutrition Examination Survey (KNHANES), we analyzed age-related variations in PhA, body composition, metabolic diseases, and nutrient intake. The mean PhA declined from 5.57° in participants aged 20–39 to 4.23° in those aged 80+, corresponding with decreases in height and weight. Individuals with higher PhA exhibited significantly greater lean body mass and lower body fat; for example, among those aged 20–39, the high PhA group showed 56.02 kg of lean mass versus 40.71 kg in the low group (p < 0.001) and total body water of 41.07 L versus 29.84 L (p < 0.001). Regarding metabolic diseases, 35.2% of the high PhA group aged 20–39 had grade 1 obesity compared with 17.4% in the low group, with similar associations observed for diabetes and hypertension across age groups. Nutritional intake was also higher in the high PhA group, with mean energy intake of 2,180.84 versus 1,710.16 kcal (p < 0.001). Overall, PhA was positively associated with favorable body composition, nutritional intake, and metabolic profiles across all age groups, suggesting its potential as a comprehensive biomarker for assessing physiological health and guiding nutritional interventions in the Korean population.