Self-management interventions in COPD aim to improve patients’ knowledge, skills and confidence to make correct decisions, thus improving health status and outcomes. myCOPD is a web-based self-management app known to improve inhaler use and exercise capacity in individuals with more severe COPD. We explored its impact in patients with mild-moderate or recently diagnosed COPD through a 12-week, 34open-label, parallel-group, randomised-controlled trial of myCOPD compared with usual care. The co-primary outcomes were between group differences in mean COPD assessment test (CAT) score at 90 days and critical inhaler errors. Key secondary outcomes were app usage and patient activation measurement (PAM) score. 3860 patients were randomised (29 myCOPD, 31 usual care). Groups were balanced for FEV1% predicted but baseline imbalance between groups for exacerbation frequency and CAT score. There was no significant adjusted mean difference in CAT score at study completion, -1.27 (95% CI -4.47 to 1.92, p=0.44) lower in COPD. However increasing app use associated with greater CAT score improvement. The odds of ≥1 critical inhaler error was lower in the myCOPD arm (adjusted odds ratio of 0.30 (0.09; 431.06, p=0.061)). The adjusted odds ratio for being in a higher PAM level at 90 days was 1.65 (0.46; 5.85) in favour of myCOPD. The small sample size and phenotypic difference between groups limited our ability to demonstrate statistically significant evidence of benefit beyond inhaler technique. However, our findings provide important insights into associations between increased app use and clinically meaningful benefit 48warranting further study in real world settings.