Purpose. In view of the increasing prevalence of overweight at early ages and its possible association with physical inactivity, investigations into the best method to assess physical inactivity and its association with excess weight in epidemiological studies are required. This study aimed to examine the associations between cardiorespiratory fitness and physical activity with indicators of adiposity in an adolescent population. Methods. This cross-sectional study involved a random sample of 697 students aged 12-19 years from public schools in the metropolitan area of Rio de Janeiro, Brazil. Overweight was classified according to body mass index. Body fat was measured by bioelectrical impedance, cardiorespiratory fitness by a 9 min run/walk test (T9), and physical activity by the International Physical Activity Questionnaire (IPAQ). Odds ratios and 95% confidence intervals (CI) were used to verify the magnitude of the associations. Results. Adolescents with poor T9 performance were more likely to be overweight (OR = 2.9, 95% CI 1.2-7.0) and have excess body fat (OR = 2.2, 95% CI 1.1-4.3) than those with better performance. Those classified as moderately active by the IPAQ were more likely to have excess body fat than those classified as active (OR = 1.8, 95% CI 1.2-2.8). Conclusions. Because of the greater magnitude of the association between cardiorespiratory fitness, as assessed by using the T9, with being overweight and having excess body fat, the T9 may serve as a valuable instrument in the school environment to identify inactive adolescents who are at risk of developing obesity.