Background Behavioural therapy for tics is difficult to access, and little is known about its effectiveness when delivered online. Objective To investigate the clinical and cost-effectiveness of an online-delivered, therapist- and parent-supported therapy for young people with tic disorders. Design Single-blind, parallel-group, randomised controlled trial, with 3-month (primary end point) and 6-month post-randomisation follow-up. Participants were individually randomised (1 : 1), using on online system, with block randomisations, stratified by site. Naturalistic follow-up was conducted at 12 and 18 months post-randomisation when participants were free to access non-trial interventions. A subset of participants participated in a process evaluation. Setting Two hospitals (London and Nottingham) in England also accepting referrals from patient identification centres and online self-referrals. Participants Children aged 9–17 years (1) with Tourette syndrome or chronic tic disorder, (2) with a Yale Global Tic Severity Scale-total tic severity score of 15 or more (or > 10 with only motor or vocal tics) and (3) having not received behavioural therapy for tics in the past 12 months or started/stopped medication for tics within the past 2 months. Interventions Either 10 weeks of online, remotely delivered, therapist-supported exposure and response prevention therapy (intervention group) or online psychoeducation (control). Outcome Primary outcome: Yale Global Tic Severity Scale-total tic severity score 3 months post-randomisation; analysis done in all randomised patients for whom data were available. Secondary outcomes included low mood, anxiety, treatment satisfaction and health resource use. Quality-adjusted life-years are derived from parent-completed quality-of-life measures. All trial staff, statisticians and the chief investigator were masked to group allocation. Results Two hundred and twenty-four participants were randomised to the intervention (n = 112) or control (n = 112) group. Participants were mostly male (n = 177; 79%), with a mean age of 12 years. At 3 months the estimated mean difference in Yale Global Tic Severity Scale-total tic severity score between the groups adjusted for baseline and site was −2.29 points (95% confidence interval −3.86 to −0.71) in favour of therapy (effect size −0.31, 95% confidence interval −0.52 to −0.10). This effect was sustained throughout to the final follow-up at 18 months (−2.01 points, 95% confidence interval −3.86 to −0.15; effect size −0.27, 95% confidence interval −0.52 to −0.02). At 18 months the mean incremental cost per participant of the intervention compared to the control was £662 (95% confidence interval −£59 to £1384), with a mean incremental quality-adjusted life-year of 0.040 (95% confidence interval −0.004 to 0.083) per participant. The mean incremental cost per quality-adjusted life-year gained was £16,708. The intervention was acceptable and delivered with high fidelity. Parental engagement predicted child engagement and more positive clinical outcomes. Harms Two serious, unrelated adverse events occurred in the control group. Limitations We cannot separate the effects of digital online delivery and the therapy itself. The sample was predominately white and British, limiting generalisability. The design did not compare to face-to-face services. Conclusion Online, therapist-supported behavioural therapy for young people with tic disorders is clinically and cost-effective in reducing tics, with durable benefits extending up to 18 months. Future work Future work should compare online to face-to-face therapy and explore how to embed the intervention in clinical practice. Trial registration This trial is registered as ISRCTN70758207; ClinicalTrials.gov (NCT03483493). The trial is now complete. Funding This project was funded by the National Institute for Health and Care Research (NIHR) Health and Technology Assessment programme (project number 16/19/02) and will be published in full in Health and Technology Assessment; Vol. 27, No. 18. See the NIHR Journals Library website for further project information.