The prevalence of disordered gambling worldwide has been estimated at 2.3%. Only a small minority of disordered gamblers seek specialist face-to-face treatment, and so a need for alternative treatment delivery models that capitalise on advances in communication technology, and use self-directed activity that can complement existing services has been identified. As such, the primary aim of this study is to evaluate an online self-directed cognitive-behavioural programme for disordered gambling (GamblingLess: For Life).The study will be a 2-arm, parallel group, pragmatic randomised trial. Participants will be randomly allocated to a pure self-directed (PSD) or guided self-directed (GSD) intervention. Participants in both groups will be asked to work through the 4 modules of the GamblingLess programme over 8 weeks. Participants in the GSD intervention will also receive weekly emails of guidance and support from a gambling counsellor. A total of 200 participants will be recruited. Participants will be eligible if they reside in Australia, are aged 18 years and over, have access to the internet, have adequate knowledge of the English language, are seeking help for their own gambling problems and are willing to take part in the intervention and associated assessments. Assessments will be conducted at preintervention, and at 2, 3 and 12 months from preintervention. The primary outcome is gambling severity, assessed using the Gambling Symptom Assessment Scale. Secondary outcomes include gambling frequency, gambling expenditure, psychological distress, quality of life and additional help-seeking. Qualitative interviews will also be conducted with a subsample of participants and the Guides (counsellors).The study has been approved by the Deakin University Human Research and Eastern Health Human Research Ethics Committees. Findings will be disseminated via report, peer-reviewed publications and conference presentations.ACTRN12615000864527; results.