IntroductionTo support the implementation of advance care planning and serious illness conversations in haematology, a previously developed conversation intervention titled ‘Advance Consultations Concerning your Life and Treatment’ (ACT) was found feasible. This study aims to investigate the effect of ACT on the quality of end-of-life care in patients with haematological malignancy and their informal caregivers.Methods and analysisThe study is a nationwide 2-arm cluster randomised trial randomising 40 physician–nurse clusters across seven haematological departments in Denmark to provide standard care or ACT intervention. A total of 400 patients with haematological malignancies and their informal caregivers will be included. The ACT intervention includes an ACT conversation that centres on discussing the patient’s prognosis, worries, hopes and preferences for future treatment. The intervention is supported by clinician training and supervision, preparatory materials for patients and informal caregivers, and system changes including dedicated ACT-conversation timeslots and templates for documentation in medical records.This study includes two primary outcomes: (1) the proportion of patients receiving chemotherapy within the last 30 days of death and (2) patients’ and informal caregivers’ symptoms of anxiety (General Anxiety Disorder-7) at 3 6, 9, 12 and 18 months follow-up. Mixed effects models accounting for clusters will be used.Ethics and disseminationThe Declaration of Helsinki and the European GDPR regulations as practised in Denmark are followed through all aspects of the study. Findings will be made available to the participants, patient organisations, funding bodies, healthcare professionals and researchers at national and international conferences and through publication in peer-reviewed international journals.Registration detailsThe study is registered at ClinicalTrials.gov (NCT05444348). The Regional Ethics Committee of the Capital Region of Denmark (record no: 21067634) has decided that approval is not necessary as per Danish legislation. Study approval has been obtained from The Capital Region of Denmark Data Protection Agency (record no: P-2022-93).Trial registration numberNCT05444348.