BACKGROUND Despite the well-known benefits of physical activity (PA) on overall health, the majority of the adult population does not engage in sufficient PA. To develop effective interventions to increase PA, it is necessary to understand the most important PA correlates and to investigate whether correlates are similar in different population subgroups. OBJECTIVE This study examined associations between physical environmental perceptions and self-reported and objectively assessed PA in Belgian adults. Moreover, associations between psychosocial factors and PA, and the moderating effects of sociodemographic factors were investigated. METHODS A sample of 1,200 Belgian adults (20-65 years; 47.9% males) completed a survey measuring sociodemographic variables and psychosocial correlates, the Neighborhood Environmental Walkability Scale and the long-version International Physical Activity Questionnaire. They wore an accelerometer for 7 days. RESULTS Perceiving neighborhoods to be high walkable (high residential density, high land use mix access, and high land use mix diversity) and recreation facilities to be convenient, and the availability of home PA equipment were the most consistent physical environmental correlates of PA. The strongest psychosocial correlates were social support from friends and family and self-efficacy. The psychosocial associations were most consistent for self-reported leisure-time PA and less clear for self-reported active transportation and accelerometer-assessed PA. Few significant sociodemographic moderators were found. CONCLUSIONS Both physical environmental and psychosocial factors were associated with PA in adults, with psychosocial factors being important especially for leisure-time PA. Correlates of PA were similar regardless of gender, age, or socio-economic status, so interventions to change these factors could have population-wide effects.