Background: Activity trackers such as the Fitbit Charge 2 enable users and researchers to monitor physical activity in daily life, which could be beneficial for changing behaviour. However, the accuracy of the Fitbit Charge 2 in a free-living environment is largely unknown. Objective: To investigate the agreement between Fitbit Charge 2 and Actigraph GT3X for the estimation of steps, energy expenditure, time in sedentary behaviour, and moderate-to-vigorous physical activity under free-living conditions, and further examine to what extent placing the Actigraph on the wrist as opposed to the hip would affect the findings. Methods: 48 adults (n=10 males, n=38 females ) were asked to wear a Fitbit Charge 2 device and two Actigraph GT3X devices (one on the hip and one on the wrist) for seven consecutive days and fill out a log of wear times. Agreement was assessed through Bland-Altman plots combined with multilevel analysis. Results: The difference between the hip-worn Actigraph and Fitbit was 1,385 steps/day (limits of agreement [LoA]=-2,342; 5,058) higher and -15 min/day (LoA=-158; 128) for time in sedentary lower than hip-worn Actigraph. Both Bland-Altman plots showed fixed bias. For time in moderate-to-vigorous physical activity, the difference between the two devices was -36 min/day (LoA=-142; 69) and the difference for activity energy expenditure was 377 kcal/day (LoA=-372; 1125) kcal/day. For the two latter outputs, the plots indicated a proportional bias. Similar or more pronounced discrepancies appeared when comparing to the wrist-worn Actigraph. Conclusion: Depending on the context, the Fitbit could be utilised for measuring daily steps and time in sedentary behaviour, but for time in moderate-to-vigorous physical activity and energy expenditure the discrepancy between devices should be considered. It would be beneficial both for validation studies and other research purposes if the technical details and algorithms of commercial devices were disclosed.