Background <p>Online behaviour change interventions (OBCI) have been poorly explored in adults with asthma. This study aimed to investigate the short- and medium-term efficacy of an OBCI in improving primarily physical activity (PA) and sedentary behaviour (SB), and secondarily in asthma control, quality of life (QoL), motivation, and basic psychological needs for exercise in adults with asthma.</p> Methods <p>Adults with stable asthma were assessed at baseline, post-intervention, and 6-month follow-up. PA and SB were evaluated by accelerometry. Asthma control, QoL, exercise motivation, and basic psychological needs for exercise were assessed by questionnaires. After randomization, both groups (control [CG] and intervention [IG]) received an educational program, the IG also received an OBCI for 12 weeks. This randomized controlled trial was designed to detect change in steps/day (80% power, effect size = 0.95, <i>N</i> = 46).</p> Results <p>Fifty-one participants were included (41 ± 11 years, 31 ± 6 Kg/m², FEV₁ 78 ± 20%predicted) and allocated into CG (<i>N</i> = 26) and IG (<i>N</i> = 25). Linear mixed models showed no significant change in any PA or SB variables when comparing groups post-intervention (<i>p</i> ≥ 0.19), despite the IG having improved steps/day during the intervention (<i>p</i> &lt; 0.001). IG participants increased exercise motivation and felt more competent to exercise than the CG post-intervention (<i>p</i> ≤ 0.03). More IG participants met the minimal important difference for asthma control and QoL compared to CG (<i>p</i> ≤ 0.03).</p> Conclusions <p>This OBCI had no additional value to the educational program in improving PA or SB of adults with asthma after the intervention. However, it might be beneficial for asthma control and QoL and enhance exercise motivation and competence.</p> Trial registration <p>Clinicaltrials.gov NCT05241223 on 15/02/2022.</p>