Objective: This systematic review aims to examine the roles of family members (FMs) in treatment decision-making for adult patients without cognitive or psychiatric disorders affecting their ability to participate in decision-making. Methodology: A comprehensive, systematic search of the Cochrane Library, PsycINFO, PubMed and ScienceDirect databases, with relevant keywords, was conducted. Two authors evaluated the eligibility of the studies independently, then cross-checked for accuracy. The quality of included studies were assessed using standardized criteria. Results: Out of the 12137 studies identified, 40 were included. Results highlighted the different roles and influences FMs have in the decision-making process. Moreover, several factors ranging from personal to cultural and family-related factors influence their level of involvement. Regardless of the illness, some similarities in family influence exist (e.g., social support). However, the type of family involvement varies according to the illness, the treatment choice and the patients’ culture. Conclusion: FMs have an important role in the decision-making process. In fact, the final decision is often made by the patients after consulting their families. FMs can support both patients and medical teams, and thus facilitate the process. Practice implications: Physicians should include FMs in treatment decision-making when the patients and their FMs wish to be included.