introductionAddressing physical inactivity is a promising dementia risk reduction strategy due to its direct benefits for brain health, and indirect benefits for other modifiable dementia risk factors. A potential limitation of previous interventions is that they often overlook how increasing physical activity affects other behaviours throughout the 24-hour day, such as sleep and sedentary behaviour, which are also important for brain health. Further, interventions are rarely tailored to the individual, considering their needs, preferences and constraints that may serve as barriers or facilitators to behaviour change. The current phase I randomised controlled trial, METHODS AND ANALYSIS: Participants aged ≥65 years from Adelaide, South Australia will be recruited and randomised to either the Extended or Condensed programme. During the first 12 weeks, participants in the Extended programme will use a tailored website to set personalised weekly goals to move towards their 'optimal' 24-hour day for brain health, facilitated by weekly website 'check-ins' and weekly phone calls with a research staff member. Participants randomised to the Condensed programme will have access to the website educational resources only but will not undergo personalised goal setting or telephone calls. Following the introductory phase (first 12 weeks), phone calls will be gradually withdrawn for the Extended programme. Primary (feasibility and acceptability) and secondary outcomes (changes in time use, cognitive function and behaviour change metrics) will be assessed 12, 24 and 36 weeks after the beginning of the intervention. ETHICS AND DISSEMINATION: Ethics approval has been obtained from the University of South Australia's Human Research Ethics Committee (205989). Study findings will be disseminated through peer-reviewed journal articles, conference presentations, media releases and community engagement. TRIAL REGISTRATION NUMBER: NCT06291909).