Background: This study investigated the associations of subjectively and objectively measured physical activity (PA) and sedentary behavior with academic achievement. We further examined whether aerobic fitness, obesity, and bedtime mediate these associations. Methods: This study included 970 children aged 9-15 years (52.3% girls) from 9 schools throughout Finland. Register-based academic achievement [grade point average (GPA)] as well as self-reported and accelerometer-measured PA/ sedentary behavior were assessed during spring 2013. Aerobic fitness (assessed via a maximal shuttle run test), body composition (assessed via bioimpedance analysis), and self-reported bedtime were collected. Structural equation modeling was applied to examine the associations. Standardized regression coefficients are presented. Results: Self-reported PA had a direct positive [β = 0.084; 95% confidence interval (CI), 0.023 to 0.145] and an indirect positive association with GPA through higher aerobic fitness (β = 0.061; 95% CI, 0.033 to −0.087). Accelerometer-based PA was not associated with GPA. Self-reported screen time had an indirect negative association with GPA through later bedtime (β = −0.071; 95% CI, −0.096 to −0.035) and lower aerobic fitness (β = −0.039; 95% CI, −0.059 to 0.019). Nonscreen sedentary time had a direct positive (β = 0.193; 95% CI, 0.101 to −0.289) and an indirect negative association with GPA through lower aerobic fitness (β = −0.040; 95% CI, −0.063 to −0.016). Conclusions: Participating in PA, avoiding excessive screen time, and going to bed earlier may benefit academic achievement.Keywords: academic performance, moderate to vigorous physical activity, screen time, sedentary time, adolescents Globally, children spend extensive amounts of time being sedentary, 1 and only one-third of children are physically active enough to gain health benefits. 2 Accompanying these trends, the rates of childhood overweight and obesity, along with poor fitness, are escalating. 3,4 These trends have raised concerns about the effects of a physically inactive lifestyle on children's health during maturation and across the lifespan, as physical activity (PA) continues to decrease and sedentary time (ST) continues to increase from childhood to adolescence and adulthood. 5,6 The association of a physically active lifestyle with learning outcomes has also recently received considerable attention. Previous studies have suggested that excessive screen time 7,8 and excess adiposity 9,10 may predict poorer academic achievement (AA), whereas regular PA and higher aerobic fitness 11,12 benefit AA.Sufficient sleep has also been related to better AA. 13,14 The periods of sleep, together with time spent in PA and sedentary behavior (SB), have been suggested to constitute a composite whole, as the time spent in one of these behaviors necessarily displaces time spent in another. 15 Although these behaviors are closely related to each other, the associations with AA have thus far been studied only in isolation or with a partial adjustment for ...