Background: Systemic treatment for advanced cancer offers uncertain and sometimes little benefit while the burden can be high. Hence, treatment decisions require Shared Decision Making (SDM). The CHOICE trial examines the separate and combined effect of oncologist training and a patient communication aid on SDM in consultations about palliative systemic treatment. Methods: A RCT design with four parallel arms will be adopted. Patients with metastatic or irresectable cancer with a median life expectancy <12 months who meet with a medical oncologist to discuss the start or continuation of palliative systemic treatment are eligible. A total of 24 oncologists (in training) and 192 patients will be recruited. The oncologist training consists of a reader, two group sessions (3.5 h; including modelling videos and role play), a booster feedback session (1 h) and a consultation room tool. The patient communication aid consists of a home-sent question prompt list and a value clarification exercise to prepare patients for SDM in the consultation. The control condition consists of care as usual. The primary outcome is observed SDM in audio-recorded consultations. Secondary outcomes include patient and oncologist evaluation of communication and decision-making, the decision made, quality of life, potential adverse outcomes such as anxiety and hopelessness, and consultation duration. Patients fill out questionnaires at baseline (T0), before (T1) and after the consultation (T2) and at 3 and 6 months (T3 and T4). All oncologists participate in two standardized patient assessments (before-after training) prior to the start of patient inclusion. They will fill out a questionnaire before and after these assessments, as well as after each of the recorded consultations in clinical practice. Discussion: The CHOICE trial will enable evidence-based choices regarding the investment in SDM interventions targeting either oncologists, patients or both in the advanced cancer setting. The trial takes into account the immediate effect of the interventions on observed communication, but also on more distal and potential adverse patient outcomes. Also, the trial provides evidence regarding the assumption that SDM about palliative cancer treatment results in less aggressive treatment and more quality of life in the final period of life. Trial registration: Netherlands Trial Registry number NTR5489(prospective; 15 Sep 2015).