Aim and objectives: To determine the factors influencing nurses’ decisions and capacity to reduce sedentary behaviour in hospital inpatients in sub-acute hospital settings. Background: Sedentary behaviour in hospital inpatients is a complex issue that can be resistant to resolution. There is little research investigating factors influencing nurses’ promotion of reduced levels of sedentary behaviour in sub-acute hospital settings. Design: An explanatory sequential design was employed, comprising quantitative and qualitative phases. Methods: An online survey was conducted with a convenience sample of 138 nurses from five Australian states. Logistic regression modelling identified demographic and behavioural characteristics of nurses who often encouraged patients to reduce their sedentary behaviour. In-depth interviews were conducted with 11 ward nurses and nurse managers, with the content subjected to thematic analysis. STROBE and GRAMMS checklists were employed. Results: Nurses recognised their role in promoting reduced sedentary behaviour but faced a range of personal and organisational barriers in achieving this outcome for patients. Few nurses were aware of national physical activity and sedentary behaviour guidelines. Five themes emerged from interviews (nursing role, care challenges, expectations of advocates, teamwork and improving the experience). Overall, many nurses experienced a lack of agency in promoting reduced sedentary behaviour and cognitive dissonance in feeling unable to undertake this role. Conclusions: The results of this study are significant in confirming that reducing sedentary behaviour in hospital inpatients is influenced by a range of complex and multi-level factors. There is a fundamental need for organisational and clinical leadership in building a culture and climate in which staff feel empowered to promote reduced sedentary behaviour in their patients. Relevance to clinical practice: The results of this study highlight the importance of taking action to reduce sedentary behaviour in sub-acute hospital settings. A co-design approach to developing interventions in local health services is warranted.