Background: Multidomain lifestyle interventions for dementia prevention depend on sustained behaviour change, yet the motivational drivers underlying these behaviours are rarely examined. The Health Belief Model identifies key constructs underpinning behaviour change, including perceived benefits, perceived barriers, self-efficacy, and cues to action. This secondary analysis of the BetterBrains trial (ACTRN12621000458831) examines whether coach-delivered motivational interviewing to modify dementia risk factors increased motivational drivers of health behaviour change. Methods: 1,037 participants were randomised to a 24-month, online, coach-delivered dementia risk factor management program (n=517) or a psychoeducational control (n=520). Motivation was assessed at baseline, 12 and 24 months using the Motivation to Change Lifestyle and Health Behaviours for Dementia Risk Reduction scale (MCLHB-DRR; seven subscales, z-standardised). Linear mixed-effects models tested group×time interactions, adjusted for age and rurality. Results: Between-group differences favoured the intervention on four of the seven MCLHB-DRR subscales; these included increases in self-efficacy (12m β=0.28, 0.12–0.43; 24m β=0.32, 0.18–0.47; p<0.001), perceived benefits (β=0.36, 0.22–0.50; β=0.29, 0.15–0.43; p<0.001), and at 12-months only, perceived barriers (β=-0.18, -0.32 to -0.05, p=0.02) and cues to action (β=0.26, 0.11–0.40 p=0.002). There were no effects for perceived susceptibility, severity, or general health motivation. Conclusion: Results suggest that participation in the BetterBrains intervention increased motivational drivers of behaviour change but did not modify risk appraisal constructs. Sustained effects on self-efficacy and perceived benefits beyond active coaching suggest durable motivational change. Findings support the hypothesised mechanism of coach-delivered motivational interviewing and identify motivational drivers of behaviour change as modifiable intervention targets for engaging midlife adults in dementia risk reduction behaviours.