ObjectiveDepressive symptoms are common among people with dementia, impacting quality of life and cognitive and functional decline. Currently, little is known about the acceptability and feasibility of psychological interventions for people with mild dementia, with recent reviews identifying the need for further evidence. Developing and evaluating psychological interventions to prevent and treat these symptoms is, therefore, an important clinical and research priority. This protocol describes a study testing the acceptability and feasibility of a manual-based behavioural activation (BA) intervention for preventing and treating depressive symptoms in people with mild dementia. The aim of this study is to explore the feasibility of conducting a pragmatic multicentre randomised controlled trial of clinical effectiveness of an eight-session intervention. The Intervention to prevent Depressive symptoms and promote well-being in EArly-stage dementia (IDEA) programme supports people with dementia and their family carers in identifying and scheduling enjoyable and meaningful activities.Methods and analysisSixty people who have received a diagnosis of dementia of any type in the last 6 months will be recruited via memory clinics. Further criteria are a Mini-Mental State Examination score of ≥20, and a family carer who can assist with the intervention. Consenting participants will be randomised in a ratio of 2:1 to BA or to treatment as usual. Analyses will estimate parameters such as rates of recruitment, retention and number of sessions completed. Questionnaires measuring depressive symptoms and quality of life for both the person with dementia and their carer will be completed at baseline, 3 and 6 months. Qualitative interviews will explore acceptability of the intervention, study procedures and experiences of the sessions.Ethics and disseminationThis study received a favourable ethical opinion from the London Camberwell St Giles Research Ethics Committee (16/LO/0540). We will disseminate findings at key conferences, the Alzheimer’s Society and University College London websites and local stakeholder events.Trial registration numberISRCTN75503960; Pre-results.