BackgroundNon-suicidal self-injury (NSSI) is a growing healthcare problem. Individuals with NSSI have an increased risk of suicidality. Due to stigma, they may self-injure in secret, which means they might not seek help until events have escalated to include suicidal ideation or a mental disorder. Interventions delivered via mobile phone applications (apps) have been linked to reductions in self-injury. This protocol outlines a trial, which examines whether the Zero Self-Harm intervention, consisting of an app for people with NSSI, can reduce the number of NSSI episodes, suicide ideation, and depressive symptoms.MethodsThe trial will be conducted as a 6-month 2-arm, parallel-group, multicentre, pragmatic, randomized clinical superiority trial. The intervention group will receive the app and instructions on how to use it, while the control group will be allocated to a waitlist and allowed to download the app after 6 months. After inclusion, participants will be asked to complete questionnaires at baseline, 3 months, and 6 months. The primary outcome is the number of NSSI episodes during the preceding month, as measured at the 6 months follow-up with the Deliberate Self-Harm Inventory. A total of 280 participants, 140 in each arm, will be included.DiscussionThis trial will assess the effectiveness of the Zero Self-Harm intervention to reduce the number of NSSI episodes. If effective, the app will have the potential to support a large group of people with NSSI. Considering the stigma related to NSSI, the fact that the app may be used in private and anonymously might make it an appealing and acceptable option for support. The app was developed in collaboration with people with lived experiences related to current and/or previous NSSI. As a result of this, the app focuses on minimizing harm, rather than stopping NSSI. This might enhance its utilization.Trial registrationClinicalTrials.gov NCT04463654. Registered on 7 June 2020.