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High levels of physical activity (PA) may prevent the development of obesity. However, the magnitude and direction of the association between PA of various intensities, sedentary time and weight status remain unclear. Thus, we examined whether objectively measured sedentary time and PA independently predict gain in body weight, change in body weight and to examine the possibility of reverse causation.We examined the prospective associations between sedentary time, PA and body weight (BW). Baseline measurements were conducted in 2008/2009 and follow-up measurements in 2014/2015 in a random sample of the adult Norwegian population (N = 1710, 45.1% men). Moderate and vigorous intensity PA (MVPA) and sedentary time were measured by accelerometry and BW and height self-reported. We first modelled the associations between baseline sedentary time and PA with BW at follow-up. We then modelled the reverse associations (BW as the exposure) and sedentary time and PA (as outcomes) in separate models. All models were adjusted for age, sex, baseline value of the outcome, socio-economic status, alcohol consumption, smoking, monitor wear time and follow-up time.Body mass index (BMI) increased by 0.2 units (P = 0.003) between baseline and follow-up, and 46.5% of participants were either overweight (36.4%) or obese (10.1%) at baseline increasing to 49.6% (11.7% obese) at follow-up. Baseline sedentary time, MVPA and vigorous PA were not associated with BW at follow-up after adjustment for covariates. In contrast, baseline BW was inversely associated with MVPA (β = - 0.11; 95% confidence interval (CI); - 0.21, - 0.009) and VPA (β = -0.035; 95% CI; - 0.059, -0.011) in adjusted models. These associations were unchanged when BW was substituted by BMI.Baseline BW seems to determine a decrease in MVPA in healthy adult Norwegian men and women, more so than the reverse.