In recent years, cancer treatment methods have diversified, and there are increasing numbers of occasions where patients or their families are required to make increasingly complex decisions. Currently, there are no guidelines for determining the process and the individual who decides the treatment strategy for cancer patients who are unable to decide for themselves. Particularly, no report has been published on decision-making for end-of-life care in patients with intellectual disabilities. This report documents our involvement in decision-making during end-of-life care for a cancer patient with intellectual disabilities. This patient’s decision-making ability or lack thereof was determined using reliable and validated assessment scales. The collective decision to not resuscitate in case of cardiac arrest and to care for the patient in the palliative care unit was made through a multidisciplinary collaboration between the social welfare team and the hospital based on reports and guidelines from abroad. Going forward, guidelines for decision-making support for terminal ill cancer patients with intellectual disabilities and decision-making by proxies for such patients need to be established in Japan.