Introduction The prevalence of dementia is almost five times higher in people with intellectual disabilities compared with the general population. However, evidence-based treatments for this population are lacking, as most randomised controlled trials for dementia interventions have not included people with intellectual disabilities. Cognitive stimulation therapy (CST) has a robust evidence base in the general dementia population, consistently showing benefits to cognition, quality of life and being cost-effective. We are conducting a mixed-methods feasibility trial of group CST for people with intellectual disabilities and dementia, to determine if a future definitive randomised controlled trial is feasible.

Methods and analysis Fifty individuals with intellectual disabilities and dementia will be randomised to either the intervention arm (14 sessions of group CST plus treatment as usual) or the control arm (treatment as usual). Randomisation will occur after informed consent has been obtained and baseline assessments completed. Each arm will have 25 participants, with the intervention arm divided into five or more CST groups with three to five participants in each. The outcomes will be feasibility of recruitment, acceptability and adherence of the intervention, suitability of study outcome measures and feasibility of collecting resource use data. Quantitative and qualitative approaches, including semistructured interviews with group participants, carers and group facilitators, will be employed to assess these outcomes.

Ethics and dissemination This study has been approved by Essex REC (Ref: 21/EE/027) and the HRA ethical approval process through the Integrated Research Application System (IRAS ID: 306 756). We plan to publish the results in peer-reviewed journals and conferences as well as provide feedback to funders, sponsors and study participants.

Trial registration number ISRCTN88614460.