Introduction: Development of fully internet-based programs could provide a new avenue to improve access to healthcare for problem gamblers. In this project, we aim to assess the efficacy of a web-based cognitive intervention targeting inhibitory control among problem gamblers, using a randomised controlled design. As impaired inhibitory control is involved in self-regulation difficulties in behavioural addictions, it represents a particularly relevant cognitive process to target for an online psychological intervention. Methods and analysis: This will be a single-blinded, randomised, comparative therapeutic web-based, controlled trial. Up to 200 non-treatment seeking adult problem gamblers with a Problem Gambling Severity Index-recent (PGSI-recent) score ≥5 will be included. The intervention will be a computerised cognitive training program targeting inhibitory skills. The comparator, an active control, will be a computerised neutral sensorial program. Both programs will be carried out under similar conditions: biweekly online training for 6 weeks and optional telephone support will be offered to patients for debriefing. The main objective of the study is to assess the clinical efficacy of the online cognitive training program at 6 weeks, measured with the PGSI-recent. The secondary objectives are to assess the efficacy on the gambling behaviour assessed by the account-based gambling data, on the self-reported gambling practice, and on the inhibition performance at the neuropsychological level at 6, 14 and 52 weeks. We will also assess the acceptability of this program and the preferred level of guidance. Data analysis will be in intention-to-treat. Ethics and dissemination: This randomized controlled trial will be executed in compliance with the Helsinki Declaration, and was approved by the local ethics boards (Comité de Protection des Personnes) in October 2017. The findings will be published in peer-reviewed journals.