BACKGROUND: Shared decision making (SDM) is often defined as an interactive process that ensures that both patient and practitioner are actively involved in the treatment and that they share all relevant information to arrive at a mental health decision. Previous SDM interventions have found improvements in outcomes such as personal recovery, higher perceived involvement in treatment decisions and knowledge about one's disease. Still, SDM occurs less frequently in mental health care than in primary care. Electronic aids developed to support patient activation and SDM could be a promising mean to engage patients in their mental healthcare. The aim of this trial is to investigate the effects of using a smartphone app to promote patient activation and support SDM for people with schizophrenia-spectrum disorders in an outpatient treatment setting. METHODS: This randomised controlled trial will allocate participants to one of two groups: (1) Intervention group: smartphone app and TAU (treatment as usual) or (2) Control group: TAU without the smartphone app. A total sample size of 260 people with a diagnosis of schizophrenia, schizotypal or delusional disorder will be recruited from five OPUS teams (a specialized early intervention program) in Denmark between 2019 and 2020. The intervention will last for 6 months with data collection at baseline, and at 3 and 6 months. Primary outcome will be self-perceived patient activation. Secondary outcomes will be feeling of being prepared for SDM; self-efficacy; working alliance; treatment satisfaction; positive and negative symptoms; level of functioning; hope; and perceived efficacy in patient-provider interaction. Patients' and health providers' preferences in clinical decision making will be assessed. Patients' usage and perceived usefulness of the app will be explored. DISCUSSION: This study will investigate the efficacy of using the smartphone app to support people with severe mental illness in engaging in their own healthcare management. The study may provide evidence to the idea that linking client and practitioner in digital solutions can have advantages in facilitating SDM in mental health. The trial will provide new knowledge of whether a digital healthcare solution can improve patient activation and support SDM for people with severe mental illness. TRIAL REGISTRATION: ClinicalTrials.gov number: NCT03554655 Registered on: June 13, 2018.