Background: Accelerometer-based wearable devices are used for objectively estimating time spent in sedentary and physical activities. Accelerometer-based devices worn on the thigh (e.g., Fibion, ActivPAL, etc.) are increasingly being used in large research studies to assess also postures. They can accurately measure time spent in sitting, standing and walking in free-living settings. Objectives: To investigate the concurrent validity of a new Fibion accelerometer and a validated ActivPAL4 accelerometer for estimating sedentary and upright time in healthy individuals. Methods: A total of 29 healthy individuals, aged between 18 and 50 years, wore the Fibion and ActivPAL4 devices on the same thigh with a medical adhesive tape during one typical weekday. Concurrent validity of the Fibion and ActivPAL4 was assessed by comparing time spent in sitting, standing and walking using the intraclass correlation coefficient and Bland-Altman plots. Results: Intraclass correlation coefficients were ≥0.843 which indicated good to excellent validity between the two devices for measuring sedentary (sitting) and upright (standing and walking) time. Analysis of the Bland Altman plots revealed a moderate agreement for sitting, standing and walking time between both devices. No proportional bias was evident in the Bland-Altman plots. Conclusion: The Fibion demonstrated good to excellent validity in measuring sedentary and upright time compared to the ActivPAL4 in healthy individuals. Keywords: Validation study, sitting position, physical activity, wearable electronic device.