Background To investigate how cancer patients, family caregiver, and their treating oncologist view the risks and benefits of family involvement in cancer treatment decision making (TDM) or the degree to which these perceptions may differ. Patients and Methods A nationwide, multicenter survey was conducted with 134 oncologists and 725 of their patients and accompanying caregivers. Participant answered to modified Control Preferences Scale and investigator‐developed questionnaire regarding family involvement in cancer TDM. Results Most participants (>90%) thought that family should be involved in cancer TDM. When asked if the oncologist should allow family involvement if the patient did not want them involved, most patients and caregivers (>85%) thought they should. However, under this circumstance, only 56.0% of oncologists supported family involvement. Patients were significantly more likely to skew their responses toward patient rather than family decisional control than were their caregivers (P < .003); oncologists were more likely to skew their responses toward patient rather than family decisional control than caregivers (P < .001). Most respondents thought that family involvement is helpful and neither hamper patient autonomy nor complicate cancer TDM process. Oncologists were largely positive, but less so in these ratings than either patients or caregivers (P < .002). Conclusions Patients, family caregivers, and, to a lesser degree, oncologists expect and valued family involvement in cancer TDM. These findings support a reconsideration of traditional models focused on protection of patient autonomy to a more contextualized form of relational autonomy, whereby the patient and family caregivers can be seen as a unit for autonomous decision.