Background: Childhood cancer survivors (CCS) are at elevated risk of chronic health conditions. Sufficient physical activity and sleep, and limited sedentary behavior mitigate those risks, but device-based assessments of these behaviors among CCS are scarce. We described physical activity, sedentary behavior, and sleep among CCS using thigh-worn accelerometers and examined associated factors. Methods: We included participants of the 2024-2025 Swiss Childhood Cancer Survivor Study questionnaire wave who consented to wear an activPAL4 accelerometer continuously for eight days. Using ActiPASS, we derived physical activity types and intensities, sedentary behavior, and sleep. We stratified analyses by age (children: 5-17 years; adults: [≥]18 years). Results: 108 CCS (68 children, 40 adults) pariticipated. Children accumulated a median of 85 minutes/day of moderate-to-vigorous physical activity [IQR 57-107] and 578 minutes/day sedentary behavior [516-625]; adults 78 [61-100] and 603 minutes/day [522-650]. Both groups spent 4-5 hours/day in sedentary bouts [≥]30 minutes. Children slept 466 minutes/night [418-499], and adults 435 [407-488]. Only 9% of children and 20% of adults met recommendations for physical activity, sedentary behavior, and sleep simultaneously. Radiotherapy was associated with lower physical activity among children and higher sedentary time among children and adults. Older children achieved less physical activity and sleep, and had more sedentary time. Conclusion: CCS engage in extensive sedentary behavior, and especially in long uninterrupted sedentary bouts. Many sleep less than recommended. Healthcare practitioners should prioritize promoting physical activity among adolescent survivors and particularly those treated with radiotherapy, and support all survivors in improving sleep duration and reducing sedentary time.