INTRODUCTION: Physical activity (PA) has diverse benefits for physical and mental health and can reduce symptoms of mental illness. Adults with mental illness face practical, psychosocial and socioeconomic barriers to adopting and maintaining PA, and it is unclear how to effectively promote PA in this group. Supervised exercise interventions provide high support but may not promote autonomous motivation, which is important for PA maintenance. The aim of this study is to compare the effectiveness of two interventions to promote PA in adults with mental illness. METHODS AND ANALYSIS: This is a randomised controlled trial of two interventions to promote PA: (1) supervised exercise and gym membership and (2) motivational discussions and self-monitoring of PA using fitness trackers. The intervention duration is 16 weeks, including 8 weeks of weekly supervised group sessions, and 8 weeks of access to the gym or fitness tracker unsupervised. Participants are community-dwelling adults recruited from outpatient clinics of public mental health services. The primary outcome is PA adoption assessed using GENEActiv accelerometers worn continuously over 8 weeks. Secondary outcomes measured at baseline, postintervention (8 weeks) and follow-up (16 weeks), include exercise motivation, psychological distress and self-reported PA assessed using self-administered questionnaires and indicators of physical health measured by a researcher blinded to allocation (blood pressure, weight, waist circumference, 6 min walk test). Participant experiences will be assessed using qualitative focus groups with analysis informed by a theoretical model of behaviour (COM-B). ETHICS AND DISSEMINATION: Ethics approval has been obtained from the Royal Brisbane and Women's Hospital (HREC/17/QRBW/302). We plan to submit a manuscript on protocol development from pilot work, and a manuscript of the results to a peer-reviewed journal. Results will be presented at conferences, community and consumer forums and hospital grand rounds. TRIAL REGISTRATION NUMBER: ACTRN12617001017314; Pre-results.