Background: Low mobility of patients during hospitalisation is associated with adverse outcomes. To successfully change behaviours related to mobility of older hospitalised patients, we need to better understand the mechanisms underlying patient and healthcare professional behaviours. In this study, we thus assessed patient- and healthcare professional-reported intentions and behaviours related to mobility of older patients hospitalised on an acute medical ward, based on a theoretical framework - the Health Action Process Approach (HAPA) model - and on additional barriers and facilitators to mobility., Methods: We conducted a cross-sectional survey in April 2022 among patients aged ≥60 years recently hospitalised on an an acute medical ward of one of three hospitals of different language/cultural regions of Switzerland, and healthcare professionals (physicians, nurses/nursing assistants, physiotherapists) working on those wards. The survey assessed the HAPA model and additional barriers and facilitators to patient mobility at hospital, as previously identified in the literature. The target behaviour studied was "to move as much as possible during hospitalisation" for patients and "to ensure my patients move as much as possible during hospitalisation" for healthcare professionals. We conducted hierarchical linear regressions to determine factors associated with the self-reported intention to perform the behaviour and with the self-reported behaviour itself., Results: A total of 142 healthcare professionals (61 physicians, 59 nurses, 22 physiotherapists) and 200 patients (mean age 74 years) completed the survey. Patients with higher intention to move as much as possible during hospitalisation scored significantly higher on factual knowledge, outcome expectancies and risk perception. Healthcare professionals with higher intention to ensure that their patients move as much as possible during hospitalisation scored higher on action knowledge, outcome expectancies and risk perception. The more the patients reported that they moved as much as possible during hospitalisation, the higher their action knowledge and action control. The more healthcare professionals reported that they ensure that patients move as much as possible during hospitalisation, the higher they scored on factual knowledge, role perception, planning and action control., Conclusions: factual and action knowledge, self-efficacy, outcome expectancies, risk perception, planning and action control were identified as important drivers of patient- and healthcare professional-reported intentions and behaviours related to inpatient mobility. These parameters can be addressed through behaviour-change interventions and should be considered in future interventions to successfully implement practice changes, with the goal of improving mobility of older patients during hospitalisation, and thus the outcomes of this particularly vulnerable population.