Background <p>Adults with intellectual disabilities (ID) in supported care settings face significant cardiometabolic health disparities, exacerbated by barriers to physical activity (PA). Sustainable, theory-driven interventions are essential to address these challenges and promote cardiovascular health in this vulnerable population. This study evaluated the effectiveness of a novel music-visual guided PA intervention (MVgPA) in increasing moderate-to-vigorous PA time, improving self-efficacy and interest in PA, and enhancing functional exercise capacity among adults with ID in supported care settings. It also assessed the intervention’s acceptability among participants and care staff.</p> Methods <p>In a 25-week cluster-randomized controlled trial, 16 supported care settings (<i>N</i> = 238 adults with mild-to-moderate ID) were randomized 1:1 to MVgPA (<i>n</i> = 8 settings, 120 participants; staff training plus twice-weekly 75-minute music-paced, visually cued PA sessions) or usual care (<i>n</i> = 8 settings, 118 participants). The Information-Motivation-Strategy (IMS)-based MVgPA intervention targeted knowledge, motivation, and practical strategies. The primary outcome was accelerometer-measured moderate-to-vigorous PA time. Secondary outcomes included functional exercise capacity (6-minute walk test), exercise self-efficacy (Baseline Interview Questionnaire), and PA interest (visual analogue scale). Outcomes were measured at baseline, 13, and 25 weeks, and analyzed using three-level mixed-effects models accounting for clustering, repeated measures, and individual variability.</p> Results <p>No significant between-group differences emerged in moderate-to-vigorous PA time. Compared to controls, the MVgPA group showed significant improvements in functional exercise capacity at 13 weeks (Time*Group interaction coefficient, β: 20.89, 95% CI: 7.81 to 33.98, <i>p</i> = 0.002) and 25 weeks (β: 16.78, 95% CI: 2.44 to 31.12, <i>p</i> = 0.02); exercise self-efficacy at 13 weeks (β: 0.60, 95% CI: 0.10 to 1.10, <i>p</i> = 0.02) and 25 weeks (β: 0.96, 95% CI: 0.42 to 1.49, <i>p</i> &lt; 0.001), and PA interest at 13 weeks (β: 1.22, 95% CI: 0.48 to1.96, <i>p</i> = 0.001) and 25 weeks (β:1.76, 95% CI: 0.98 to 2.55, <i>p</i> &lt; 0.001). All intervention settings committed to sustaining MVgPA post-trial. The trial achieved 100% participant retention and demonstrated high acceptability (staff willingness scores: 4 to 6/6).</p> Conclusions <p>The IMS-based MVgPA intervention significantly improved functional and psychosocial outcomes associated with cardiovascular health in adults with ID, despite no increase in objectively measured PA. The intervention’s high acceptability and sustainability support its scalable implementation in supported care settings.</p> Trial registration <p>The study protocol was registered at ClinicalTrials.gov (NCT04938999) <a href="https://clinicaltrials.gov/study/NCT04938999?term=NCT04938999&amp;rank=1">https://clinicaltrials.gov/study/NCT04938999?term=NCT04938999&amp;rank=1</a> on 2021-06-25.</p>