Background: Patients with breast cancer face complex decisions regarding treatment options, and frequently involve family, friends, and other decision supporters in decision-making. A high-quality decision is one that is both informed and values-concordant. While much has been done to evaluate the quality of decision-making among patients, very little is known about how decision supporters fit within that framework. Thus, we sought to understand variations in objective knowledge among decision supporters. Methods: Patients with stage 0-II breast cancer reported to Georgia and LA SEER registries in 2014-15 and their key decision support person (DSP) were surveyed separately. DSPs were asked 4 objective knowledge questions regarding breast cancer treatment with responses dichotomized into high/low knowledge. Bivariate analyses and multivariable regression models were used to assess associations between DSP knowledge and important DSP characteristics (type of DSP, age, race, education), level of DSP engagement in 3 domains of patients’ decision-making (feeling informed about decisions, extent of and satisfaction with their involvement in decisions, and being aware of patients’ values/preferences), and patient clinical factors (stage, chemotherapy receipt, radiation receipt, type of surgery). Results: 2502 patients (68% RR) and 1203 eligible DSPs (70% RR) responded. Most DSPs were husbands or daughters. 21% were Latino, 17% were black, 20% had <high school education. Overall,53% DSPs had high objective knowledge. DSPs with high knowledge were more likely to be non-Black, non-Spanish-speaking Latinos, have a higher level of education, and report high engagement in the domains of satisfaction with their involvement in patients’ decision-making and awareness of patients’ values/preferences (all p < 0.05). After adjustment, high objective knowledge among DSPs was associated a higher level of education (OR = 2.44; 95% CI = 1.67-3.57) and high awareness of patients’ values/preferences (OR = 1.40; 95% CI = 1.06-1.84). Conclusions: Objective knowledge about breast cancer treatment was varied among DSPs of patients with breast cancer. Highly engaged DSPs (aware and satisfied) were more likely to have high knowledge, suggesting that involving DSPs may be an untapped mechanism for improving patients’ understanding of treatment options. These finding suggest that DSPs can play a role in improving decision quality in patients, and that interventions focused on decision-making may benefit from modules directed to DSPs as well as patients.