Objective Anxiety affects up to one-third of adults with asthma and is linked to poorer disease outcomes and reduced quality of life. This study evaluated the efficacy of therapist-guided, internet-delivered cognitive behavioural therapy (ICBT) versus treatment as usual plus medical education for reducing asthma-related anxiety. Methods A randomised controlled trial was conducted including 90 adult participants with anxiety related to asthma. ICBT was therapist-guided and lasted 8 weeks. The primary outcome, the Catastrophising about Asthma Scale, was assessed from pretreatment to the primary endpoint at 16 weeks. Secondary outcomes included asthma control, avoidance behaviour and quality of life. Forced expiratory volume in 1 s (FEV 1 ) was collected using a digital spirometer. Results ICBT demonstrated a significantly larger reduction in catastrophising about asthma than the control group (mean difference –18.53, 95% CI –25.54 to –11.53, p<0.001). Asthma control, avoidance behaviour, quality of life and other key outcomes improved significantly more in the ICBT group compared with controls. No changes in FEV 1 were observed. Improvements were sustained at 6 months follow-up. Conclusion ICBT effectively and safely reduces catastrophising about asthma, improves asthma control, avoidance behaviour and quality of life and represents a promising adjunct to routine medical care for patients with asthma complicated by anxiety. Trial registration number Clinicaltrials.gov (ID: NCT04230369 ).