Background People with schizophrenia die almost 20 years earlier than the general population, most commonly from avertable cardiometabolic disease. Existing pharmacological weight-loss agents including metformin have limited efficacy. Recently available glucagon-like peptide (GLP-1) receptor agonists such as semaglutide have shown promise for weight loss but have yet to be trialled in this population. Aims To examine the efficacy of semaglutide to ameliorate antipsychotic-induced obesity in people with schizophrenia who have been treated with clozapine for more than 18 weeks. Method This is a 36-week, double-blinded, randomised placebo-controlled trial. We will recruit 80 clozapine-treated patients with schizophrenia or schizoaffective disorder, aged 18–64 years, with a baseline body mass index ≥26 kg/m², who will be randomised to subcutaneous semaglutide of 2.0 mg once a week or placebo for 36 weeks. The primary endpoint will be percentage change in body weight from baseline. Results This trial will assess the efficacy and side-effects of the GLP-1 receptor agonist semaglutide on body weight and provide evidence on the possible clinical utility of semaglutide in patients with inadequate response to metformin. The study is registered with the Australian New Zealand Clinical Trials Registry (www.anzctr.org.au) with clinical trial registration number ACTRN12621001539820. Conclusion This research could benefit individuals with schizophrenia who experience significant health issues, leading to premature mortality, owing to antipsychotic-induced weight gain. Study findings will be disseminated through peer-reviewed publications and conference presentations.