ABSTRACT Objective: To identify sociodemographic, physical activity, and physical fitness factors associated with flexibility. Methods: Cross-sectional study with 909 adolescents (486 girls) aged 14 to 19 years from the city of São José, Santa Catarina, Brazil. To evaluate flexibility, we used the sit and reach test. Sociodemographic and physical activity data were collected by a self-administered questionnaire. We analyzed muscle strength with manual dynamometry. Aerobic fitness was assessed with the modified Canadian aerobic fitness test, and physical activity with a questionnaire. We used multiple linear regression to identify the variables associated with flexibility, with a significance level of 5%. Results: For each additional centimeter in the girls’ levels of flexibility, the boys were 2.94 cm less flexible. In addition, the increment of 0.12 kg/force in muscle strength levels increased the levels of flexibility in 1 cm. Conclusions: Lower levels of flexibility were associated with male adolescents and with reduced strength levels.