Aim Physical activity (PA) has traditionally been measured wearing accelerometers on the hip, but they are increasingly being worn on the wrist. We compared hip and wrist accelerometers with regard to their acceptability and any associations between PA and fatness and fitness. Methods This cross-sectional study comprised 103 children aged 8-12 years (62% boys) who participated in the ActiveBrains trial by the University of Granada, Spain, in 2014-2016. The children wore both ActiGraph GT3X+ hip and wrist accelerometers round the clock for 7 days. The acceptability of both placements was evaluated by a questionnaire, while the childrens fat mass index, waist circumference and cardiorespiratory fitness (CRF) were assessed. Results Wearing wrist accelerometers caused less disturbance, mainly because hip accelerometers caused more issues during the night. The measurements from both placements showed that lower PA levels were associated with fatness and that higher PA levels were associated with better CRF. Conclusion Both placements showed consistent results with regard to measuring associations between PA levels and fatness and fitness. However, wearing them on the wrist caused less discomfort at night. Future studies are needed to confirm the best placement for accelerometers during PA studies.