INTRODUCTION: In the UK, National Health Service (NHS) guidelines recommend that informal carers of people living with dementia should be offered training to help them develop care skills and manage their own physical and mental health. The WHO recommends access to affordable, proven, well-designed, online technologies for education, skills training and support for dementia carers. In response to these recommendations, this multisite randomised controlled trial (RCT) is the first study in the UK to evaluate the clinical and cost-effectiveness of an online support programme developed by the WHO called 'iSupport for dementia carers'.

METHODS AND ANALYSIS: 350 informal carers (age 18+ years) living in Britain who self-identify as experiencing stress and depression will be recruited. They will be randomised to receive 'iSupport', or standardised information about caring for someone with dementia (control-comparison). Data will be collected via videoconferencing (eg, Zoom) or telephone interview at baseline, 3 months and 6 months. Intention-to-treat analysis will ascertain effectiveness in the primary outcomes (distress and depression) and combined cost, and quality-adjusted life-year data will be used to assess cost-effectiveness compared with usual care from a public sector and wider societal perspective. A mixed-methods process evaluation with a subgroup of carers in the intervention (~N=50) will explore the barriers and facilitators to implementing 'iSupport'. A non-randomised feasibility study will adapt 'iSupport' for young carers (n=38 participants, age 11-17 years).

ETHICS AND DISSEMINATION: The research plan was scrutinised by National Institute for Health Research reviewers ahead of funding being awarded. Ethical approval was granted by Bangor University's School of Health and Medical Sciences Academic Ethics Committee, reference number 2021-16915. Dissemination plans include delivering events for stakeholders, social media, a project website, developing policy briefings, presenting at conferences and producing articles for open access publications.

TRIAL REGISTRATION NUMBER: ISRCTN17420703.