INTRODUCTION: Prolonged grief disorder (PGD) represents a substantial public health issue, especially in oncology settings where it affects up to 30% of bereaved carers. Current best-practice treatments are lengthy, and up to 50% of participants have persistent PGD. Building on encouraging recent research with psychedelic-assisted therapies, the Psilocybin-Assisted suppoRtive psychoTherapy IN the treatment of prolonged Grief (PARTING) trial is the first study to consider psilocybin-assisted psychotherapy as a potential treatment for prolonged grief. METHODS AND ANALYSIS: PARTING is an open-label pilot trial of psilocybin-assisted psychotherapy for approximately 15 people with cancer-related PGD. It aims to investigate feasibility, safety, acceptability, participant experience and participant-reported therapeutic effects. Over a 5-week intervention period, participants will undergo three preparation sessions before receiving a psychoactive (25 mg) dose of psilocybin alongside non-directive supportive guidance, followed by four integration sessions. All sessions will be delivered by a psychologist and either a nurse or Indigenous Therapist. An artificial intelligence-assisted tool will be used to create an artwork of participants' psychedelic experience.Outcomes will be investigated over a 12-month follow-up period. Feasibility will be assessed through recruitment/retention rates and completion of follow-up assessments. Safety will be evaluated via adverse events over 12 months and the comparison of physiological measures (vital signs, biochemistry, haematology, ECG) recorded during screening and 1 day after the psilocybin dose. Qualitative thematic analysis of semistructured interviews with participants and trial therapists will assess acceptability and the therapeutic potential of the treatment. Diagnostic clinical interviews for PGD and quantitative participant-reported measures of therapeutic effects are also being collected. Participant-reported measures include grief severity, depression, anxiety, grief avoidance, psychological flexibility, connectedness, and quality of life. ETHICS AND DISSEMINATION: Ethics approval has been obtained from QIMR Berghofer Medical Research Institute Human Research Ethics Committee (P3801). Dissemination of results will occur via conference presentations, peer-reviewed publications and media. TRIAL REGISTRATION NUMBER: Australian New Zealand Clinical Trials Registry (ACTRN12623000827639).