The Rare Dementia Support (RDS) Impact study will be the first major study of the value of multicomponent support groups for people living with or supporting someone with a rare form of dementia. The multicentre study aims to evaluate the impact of multicomponent support offered and delivered to people living with a rare form of dementia, comprising the following five work packages (WPs): (a) longitudinal cohort interviews, (b) theoretical development, (c) developing measures, (d) novel interventions, and (e) economic analysis. This is a mixed-methods design, including a longitudinal cohort study (quantitative and qualitative) and a feasibility randomised control trial (RCT). A cohort of more than 1000 individuals will be invited to participate. The primary and secondary outcomes will be in part determined through a co-design nominal groups technique prestudy involving caregivers to people living with a diagnosis of a rare dementia. Quantitative analyses of differences and predictors will be based on prespecified hypotheses. A variety of quantitative (eg, analysis of variance [ANOVA] and multiple linear regression techniques), qualitative (eg, thematic analysis [TA]), and innovative analytical methods will also be developed and applied by involving the arts as a research method. The UCL Research Ethics Committee have approved this study. Data collection commenced in January 2020. The study will capture information through a combination of longitudinal interviews, questionnaires and scales, and novel creative data collection methods. The notion of "impact" in the context of support for rare dementias will involve theoretical development, novel measures and methods of support interventions, and health economic analyses. © 2019 The Authors. International Journal of Geriatric Psychiatry published by John Wiley & Sons Ltd.