Higher levels of physical activity have been associated with better asthma clinical control.Does a behaviour change intervention aimed at increasing physical activity change asthma clinical control, physical activity, sedentary time, health-related quality of life (HRQoL) and anxiety and depression symptoms?and methods: Single-blind randomised controlled trial in which participants were allocated to an intervention group (IG) or a control group (CG). Both groups received usual care and disease-specific education. Participants in the IG also underwent an 8-week behaviour change intervention aimed at increasing physical activity. Before and after the intervention period, measures were made of: asthma clinical control (Asthma Control Questionnaire [ACQ]), physical activity, sedentary time and sleep quality (Actigraph), HRQoL (Asthma Quality of Life Questionnaire) and anxiety and depression symptoms (Hospital Anxiety and Depression scale). Data on asthma exacerbations were recorded 12 months before and throughout the intervention period.Fifty-one participants were included (CG=26; IG=25). On completion of the intervention period, compared to the CG, those in the IG demonstrated improvements in asthma control (mean difference [95% CI] in ACQ score -0.8 [-1.1 to -0.4]), in daily step count (3,605 [1,937 to 8,867] steps/day), in sleep efficiency (9.2 [-7.1 to 21.9%] and a reduction in sedentary time (-1.1 [-2.9 to -0.6] hours/day). No between-group difference in HRQoL was observed. The percentage of participants who exacerbated during the intervention period was 27% in the IG vs. 60% in the CG (p=0.04). The change in time spent in moderate-intensity physical activity was inversely associated with change in ACQ (r=-0.60). Compared with the CG, a higher percentage of participants in the IG reported a reduction in anxiety symptoms (43% vs. 0%; p<0.02).In adults with moderate to severe asthma, a comprehensive behaviour change intervention that increased physical activity also produced improvements in asthma clinical control, sedentary time, sleep quality and anxiety symptoms.Protocol registration and Results System, Clinical Trials.gov, NCT-03705702.