Background The need for a Core Outcome Set to evaluate physical activity interventions for people living with dementia, across stages of disease and intervention settings has been established. This qualitative study precedes the consensus phase of developing this Core Outcome Set and aims to: (i) compare the outcomes identified by patients, carers and professionals to those previously reported in the literature; (ii) and understand why certain outcomes are considered important. Design and methods In-depth semi-structured interviews were conducted with people living with dementia, family carers and professionals (n = 29). The outcomes identified in the interviews were mapped to a list of outcomes reported in a recent literature review. An in-depth thematic analysis was conducted to understand the importance of physical activity in dementia care. Results A comprehensive, inductively derived list of 77 outcomes, common across stages of dementia and intervention setting, was put together for the consensus phase of this Core Outcome Set: ten of these were new outcomes generated by this qualitative study. Five themes explained why stakeholders perceived physical activity outcomes as important for people living with dementia: "being well and staying well", "having a role towards others", "maintaining identity", "being connected to the present" and "delivering good quality care". Conclusion Ten new outcomes of physical activity, not previously reported in recent literature, were identified. Physical activity is considered important to people living with dementia due to its positive impact on multiple health outcomes for both patients and carers.