Background Nutrition is an important predictor of an athlete's performance. It is more important for particularly those athletes who are in the growing stage (adolescents). This study aimed to assess their current level of knowledge, attitude, and practices (KAP) of nutrition and to predict potential factors impacting their level of these KAP. Methods A cross-sectional study was conducted among adolescent trainee athletes selected conveniently from 11 sports departments (e.g. cricket, football, hockey, etc.) of Bangladesh Krira Shikkha Protishtan (BKSP). A semi-structured interviewer-administered questionnaire was used for data collection. Anthropometric measurements (e.g. height, weight, BMI, BF %) were taken using appropriate methods. The data were analyzed using R (v3.6.1) and Python Jupyter Notebook. Descriptive statistics, t-test, analysis of variance (ANOVA), and logistic regression were used to determine the relationship between dependent and independent variables. Results A total number of 260 participants were enrolled in this study, most of them (86%) were male. Their mean age was 15.50 ± 1.83 ranging from 12-19 years. More than half of the participants had good nutrition knowledge (n = 149, 57.3%). Fifty-seven percent of participants had a positive attitude (n = 146) and 57.69% (n = 150) had a good level of practices regarding nutrition. Age (p = 0.007), gender (p = 0.004), department of training (p = 0.0034), and duration of sports training (p = 0.004) of participants were significantly associated with knowledge, while only age and BMI were significantly associated with practices. Athletes with less body fat were more likely to have good nutrition practice behavior (AOR 0.895; 95% CI 0.83, 0.96; p < 0.05) and nutrition knowledge was positively associated with nutrition practice (AOR 2.335; 95% CI 1.405, 3.88; p = 0.001). Conclusion Knowledge, attitude, and practices level observed among many of our participants was satisfactory. Previous nutrition training was found as a potentially modifiable factor of good nutrition knowledge and good nutrition knowledge was found to be a predictor of good practice score. Thus, it is necessary to provide appropriate nutrition information to adolescent athletes through proper educational training and intervention on a regular basis.