The purpose of this study was to examine the association between sedentary behavior and obesity among 12-year-old children, while adjusting for moderate-to-vigorous physical activity (MVPA) and other potential confounding variables. Cross-sectional analyses were carried out with data from 5,434 children who participated in the Avon Longitudinal Study of Parents and Children (ALSPAC). Fat mass was derived using dual-energy X-ray emission absorptiometry, and height and weight measurements were used to calculate BMI (kg/m2). The children wore an accelerometer for 7 days. The cut points for sedentary behavior and MVPA were ≤199 and ≥3,600 counts per minute (cpm), respectively. Logistic regression analyses were performed to estimate odds ratios (ORs), adjusting for potential confounders of physical activity that included gender, social factors, early life factors, and maturation. The minimally adjusted association between sedentary behavior and obesity was positive, OR = 1.18 (1.08, 1.28). After adjusting for the series of potential confounders of physical activity the positive association remained, OR = 1.32 (1.14, 1.53). The crude association between 15 min of MVPA per day and obesity was negative, OR = 0.54 (0.48, 0.62). When 15 min of MVPA per day was additionally controlled for in the models, the positive associations between sedentary behavior and obesity were negated. Sedentary behavior was positively associated with obesity in the 12-year-old children, but this association was not independent of MVPA; low levels of MVPA among the sedentary children increased the odds of obesity. These findings support the importance of specifically engaging in MVPA during childhood to reduce the prevalence of obesity.