Purpose <p>This study examined the association between weight status and myopia in children and adolescents and compared the screening performance of body mass index (BMI) and body fat percentage.</p> Methods <p>This cross-sectional study included 8748 students aged 7–15 years. Height, weight and body fat percentage were measured, and myopia was assessed using non-cycloplegic autorefraction. The sample was randomly divided into training and validation sets. Logistic regression adjusted for sex, age, residence, physical activity and outdoor light exposure was used to examine the association between weight status and myopia. Mediation analysis treated physical activity as a categorical variable, with bootstrap resampling used to test indirect effects through BMI and body fat percentage. Receiver operating characteristic curves compared the screening performance of both indicators.</p> Results <p>Overweight or obesity was associated with myopia after full adjustment (odds ratio = 1.45, 95% confidence interval: 1.30–1.63). Categorical mediation analysis showed a direct protective association of higher physical activity with myopia, with no significant indirect effect through BMI or body fat percentage. In both training and validation sets, the area under the curve was 0.76 for BMI and 0.76 for body fat percentage. A body fat percentage threshold of 0.69 classified a high-risk group with higher myopia prevalence than the low-risk group (88.7% versus 52.1%).</p> Conclusion <p>Overweight or obesity is independently associated with myopia. BMI and body fat percentage provide comparable, low-cost tools for initial school-based risk stratification, while physical activity may protect against myopia independently of body composition.</p>