Objective: To assess the relationships among physical activity, measured objectively, and attention capacity in European adolescents. Study design: The study included 273 adolescents, aged 12.5-17.5 years, who participated in the Healthy Lifestyle in Europe by Nutrition in Adolescence Study. Participants wore a uniaxial accelerometer for 7 days to measure physical activity. The d2 Test of Attention was administered to assess attention capacity. Multivariate analyses were used to study the association of attention capacity with each measure of physical activity. Receiver operating characteristic analysis was performed to determine thresholds that best discriminate between low and good attention capacity. Results: After controlling for potential confounding variables (age, sex, body mass index, parental educational level, fat mass, aerobic fitness, and center), adolescents' attention capacity test performances were significantly and positively associated with longer time spent in moderate or moderate-to-vigorous physical activity (MVPA) in free-living conditions (P < .05). Receiver operating characteristic curve analyses revealed that the physical activity thresholds that best discriminated between low/good attention capacities were ≥41 min·day−1 for moderate, ≥12 min·day−1 for vigorous, and ≥58 min·day−1 for MVPA. Conclusion:These findings suggest that promoting MVPA may be have a beneficial effect on attention capacity, an important component of cognition, in adolescents.