Limited data are available regarding the physiological profile of youth triathletes. The aim of this study was to characterize the physiological and body composition profile of Hungarian youth triathletes and to examine the relationships between anthropometric characteristics and aerobic performance indicators. Forty-one youth triathletes (20 females and 21 males; age: 15.8 ± 1.7 years), members of the Hungarian national development squad, participated in the study. Anthropometric and body composition parameters were assessed using standardized procedures and multi-frequency bioelectrical impedance analysis. Aerobic performance was evaluated using a graded cardiopulmonary exercise test on a treadmill with breath-by-breath gas analysis. Male athletes demonstrated higher body height, body mass, fat-free mass, and skeletal muscle mass compared with females (p < 0.05). Cardiopulmonary exercise testing revealed high aerobic capacity, with mean VO2max values of 73.2 ± 5.4 mL·kg⁻¹·min⁻¹ in males and 63.1 ± 5.0 mL·kg⁻¹·min⁻¹ in females. The second ventilatory threshold occurred at approximately 82–86% of VO2max. Strong positive correlations were observed between anthropometric parameters and absolute oxygen uptake (mL·min⁻¹), particularly for fat-free mass, skeletal muscle mass, and body surface area (r = 0.83–0.95). However, these relationships are influenced by body size and were weaker or inverse when relative oxygen uptake (mL·kg⁻¹·min⁻¹) was considered. Regression analyses further indicated that body composition variables, especially fat-free mass and skeletal muscle mass, were positively associated with aerobic performance, while body fat percentage was not a significant predictor when body size and sex were controlled. These findings are based on cross-sectional associations and should be interpreted as descriptive reference data for this population rather than predictive criteria. The results contribute to the characterization of physiological and anthropometric profiles in youth triathletes and may support future research and athlete monitoring.