Background Family caregivers often play a key role in medical decision-making for patients with cancer. Adult-children account for nearly half of caregivers, but often have less experience with serious illness care and decision-making and face unique relational challenges as the patient’s child. Yet little research explores the potentially distinctive decision-making, involvement in decisions, and support needs of adult-child caregivers. Methods Analysis of survey data of U.S. cancer caregivers conducted by CancerCare® in 2021. Chi-square tests and multivariable regression models assessed whether adult-child and spousal caregivers differed on the type of medical decisions they participated in (e.g., treatment planning, medication management), who made the decision (e.g., caregiver or joint decision), and the resources that informed decisions (e.g., friends and family, education materials). Results Adult-children (N = 892) were less likely than spouses (N = 314) to participate in treatment planning (beta = -0.41; 95%CI = -0.81,-0.01), but more likely to be involved in decisions about whether to challenge medical authority (e.g., seeking alternative treatment, second opinion) (beta = 0.50; 95%CI = 0.22,0.78). Compared to spouses, adult-children made joint decisions with patients less often (-13.2-percentage points; 95%CI = -19.64,-6.67) and acted as primary decision-maker more frequently (5.60-percentage points; 95%CI = 0.01,10.43). More adult-children than spouses sought help and information regarding decisions from the oncology team (8.42-percentage points; 95%CI = 1.98,14.87) and friends and family (7.91-percentage points; 95%CI = 1.34,14.48). Conclusions How cancer caregivers and patients are related to each other shapes caregivers’ medical decision-making. Adult-children’s and spouses’ probabilities of participating in and influencing decisions differed for certain types of decisions while adult-children were more likely to seek information and social support regarding decisions. These findings highlight the importance of the patient’s and caregiver’s relationship type in medical decision-making, suggesting that decision support programs may be more effective if they tailor programs by relationship type.