The benefits of physical activity (PA) on children's health and well-being are well established. However, many children do not meet the PA recommendations, increasing their risk of being overweight, obese, and non-communicable diseases. Environmental characteristics of homes and neighborhoods may constrain a child's ability to engage in PA, but evidence is needed to inform country-specific interventions in understudied low-income countries. This study assessed the associations between parental-perceived home and neighbourhood, built environment characteristics, and moderate-to-vigorous physical activity (MVPA) among children in Kampala city, Uganda. In this cross-sectional study, data were obtained from 256 children (55.5% girls) aged between 10 and 12 years and their parents. Children's MVPA was measured using waist-worn ActiGraph accelerometers. The environments were assessed using a valid self-reported parent survey. Linear regression models with standard errors (clusters) were used to analyze the relationship between environmental variables and children's MVPA. Sex-specific relationships were assessed using sex-stratified models. Play equipment at home (β = -2.37, p <0.001; unexpected direction), residential density (β = 2.70, p<0.05), and crime safety (β = -5.29, p <0.05; unexpected direction) were associated with children's MVPA. The sex-specific analyses revealed more inconsistent patterns of results with a higher perception of land use mix associated with less MVPA in girls (irrespective of school type attended), and higher perceptions of sidewalk infrastructure (β = -12.01, p <0.05) and walking and cycling infrastructure (β = -14.72, p <0.05) associated with less MVPA in girls attending public schools only. A better perception of crime safety was associated with less MVPA among boys and girls attending private schools (β = -3.80, p <0.05). Few environmental characteristics were related to children's MVPA in Uganda, and findings were largely inconsistent, especially among girls. Future studies are needed to understand the ecological determinants of health-related PA behaviors among children in Uganda.