SummaryBackgroundNIDUS-Family is a 6-8 session, psychosocial and behavioural intervention, delivered by non-clinical facilitators, tailored to goals set by dementia-unpaid/family carer dyads. It is effective in terms of attainment of personalised client goals. We aimed to determine if it is cost-effective.MethodsThis cost utility and cost-effectiveness analysis is within a two-armed, single masked, multi-site, superiority Randomised Controlled Trial (RCT). We recruited 302 dyads from community settings. Randomisation was blocked and site-stratified, using a 2:1 ratio (intervention: control (goal-setting and routine care)), with allocation by remote web-based system. We calculated the probability that NIDUS-Family is cost-effective for a client with dementia based on Quality Adjusted Life Year (QALY) from health and personal social services and societal perspectives, at £20,000-£30,000 decision thresholds for QALY gained, compared to usual care over 12 months. Analyses were intention-to-treat.Trial registration:ISRCTN11425138.FindingsFrom 30.4.2020-9.5.2022, 204 participants (109 (53.4%) female) were randomised to intervention and 98 (60 (61.2%) female) to control. 218 (72.2%) participants at 6 months and 178 (58.9%) at 12 months provided cost data. There was 89% and 87% probability that NIDUS-Family was cost-effective compared to usual care from personal social services and societal perspectives respectively. Intervention participants accrued on average £8934 (37%) less costs than control participants (95% CI -£59,460 to £41,592).InterpretationNIDUS-Family is the first personalised care and support intervention to demonstrate cost-effectiveness from the perspective of the quality of life of people with dementia, as well as clinical effectiveness and should be part of routine dementia care.FundingThis work was supported by the Alzheimer’s Society (Centre of Excellence grant 330).