Many older patients with cancer depend on their family members for care and support and involve their family members in treatment decision-making in different stages of the cancer trajectory. Although family involvement is advocated in person-centered care, little is known about family involvement in decision-making specifically for older patients, and evidence-based strategies are scarce. The aim of this scoping review is to provide deeper understanding of factors influencing family involvement in treatment decision-making for older patients with cancer. Four databases were searched for quantitative-, qualitative- and mixed-method empirical studies describing factors influencing family involvement in treatment decision-making for older patients with cancer: PubMed, EMBASE, CINAHL and PsycINFO. Three independent researchers reviewed the papers for eligibility and quality and contributed to the data extraction and analysis. Twenty-seven papers were included, sixteen quantitative studies, nine qualitative studies and two mixed-method studies. Five categories of factors influencing family involvement emerged: 1) patient characteristics, 2) family member characteristics, 3) family system characteristics, 4) physician's role and 5) cultural influences. These factors affect the level of family control in decision-making, treatment choice, decision agreement, and levels of stress and coping strategies of patients and family members. This review reveals a complex interplay of factors influencing family involvement in treatment decision-making for older patients with cancer that is rooted in characteristics of the family system. The findings underscore the need for development and implementation of evidence-based strategies for family involvement in treatment decision-making as part of patient-centered care for older patients with cancer.