The decision to withhold or withdraw life-sustaining treatment (LST) lies sometimes in the hands of intensive care unit patients' families. Since 2018, family members of dying patients in South Korea have had the legal right to make decisions for the patients. This study aimed to examine knowledge and attitude toward LST among the family members of patients under intensive care since the foundation of the new legislation. Their perceptions of the roles of nurses were also explored. In this cross-sectional study, 89 participants completed survey questionnaires on demographic characteristics, relationship to patient, reason for admission, length of stay, awareness of the new legislation, knowledge and attitude toward LST, and perception of the role of nurses. The results indicated that knowledge was significantly associated with attitude and was, in fact, the only predictor of attitude toward LST (
P
= .021). Explaining the disease and prognosis in detail and relieving patients of their physical pain were the roles of nurses most valued by family members. The findings suggest that it is crucial for nurses to support family members and provide information about critical changes and medical options for collaborative decision making according to the patients' wishes during the dying process.