Background: Background: Hospitalized frail older adults have an increased risk of developing hospital acquired disability associated with hospital practices of restricted physical activity and immobilization. The use of activity tracking wearable devices may allow to identify and prevent mobility decline reducing hospital acquired disability. Objective: Objectives: To identify the optimal wearable device and wear location for monitoring mobility in older hospitalized patients. Specific objectives included: 1) compare the feasibility and acceptability of ActiGraph wGT3X-BT, MOX1, MetaMotionC, and Fitbit Versa for continuous mobility monitoring, and 2) determine the concurrent validity of the selected device for detecting body posture and step count. Methods: Methods: Participants were recruited for this observational study in the acute medical care unit of an academic hospital in Hamilton, Ontario, Canada. Eligible patients were aged 60 years and older, able to undertake the mobility protocol, and had an anticipated length of stay greater than 4 days. The study was divided into two experiments. Experiment 1evaluated the feasibility of four wearable devices and validated the derived data for body posture and step count. Experiment 2 involved a mobility assessment session and a 24-hour monitoring period with the selected device from Experiment 1. Results: The ActiGraph wGT3X-BT emerged as the most feasible device, demonstrating superior usability, data acquisition, and management. The thigh-worn ActiGraph accurately detected sedentary behavior, while the ankle-worn device provided detailed information on step counts and body postures. Bland-Altman plots and Intraclass Correlation Coefficients (ICCs) indicated that the ankle-worn ActiGraph showed excellent reliability for step counting, with minimal bias and narrow limits of agreement. Patients expressed a high willingness to wear a continuous mobility tracking device at the hospital and at home. Conclusions: Thigh and ankle-worn ActiGraph are optimal for assessing and monitoring mobility in older hospitalized patients. These findings support the integration of wearable technology in hospital settings to enhance mobility monitoring and early intervention strategies. Further research is warranted to evaluate the long-term use of wearable data for predicting health outcomes post-hospitalization and informing clinical decision-making to promote early mobility.