OBJECTIVE Individuals may achieve high physical activity (PA) yet also be highly sedentary (SED). This study assessed adiposity in children classified by PA/SED groups. METHODS Participants were 520 8- to 10-year-old children with ≥ 1 obese parent. Moderate-to-vigorous PA (MVPA) and SED were measured by accelerometer, and screen-time was measured by self-report. Height, weight, waist circumference (WC), body fat percentage (BF%), and VO2peak were objectively measured; energy intake was measured by dietary recall. Elevated adiposity was defined as BMI ≥ 85th percentile, WC ≥ 90th percentile, BF% ≥ 85th percentile, or waist-to-height ratio (WHR) ≥ 0.5. RESULTS Up to 27% of boys and 15% of girls were active/SED. Adiposity was lowest for active/non-SED, highest for inactive/SED, and intermediate and similar for active/SED and inactive/non-SED. Using 60 min/d MVPA and 2 h/d screen-time cut-offs, prevalence ranges for elevated adiposity in the active/non-SED, active/SED, inactive/non-SED, and inactive/SED groups were 0% to 14%, 15% to 44%, 16% to 40%, and 32% to 51%, respectively. Corresponding odds and 95% confidence intervals of being overweight/obese for the latter groups were 3.8 (95% CI, 1.7-8.4), 3.8 (1.8-8.2), and 4.9 (2.3-10.3) versus active/non-SED. PA/SED-adiposity associations were mediated by fitness but not energy intake. CONCLUSIONS Combined PA/SED levels are strongly associated with adiposity in children, but associations are mediated by fitness. Active children who accumulate >2 h/d of screen time and inactive children are equally likely to be overweight/obese.