Family caregivers are increasingly recognized as a vital part of the comprehensive treatment of cancer. Many caregivers, especially those caring for patients with primary malignant brain tumor (PMBT), report feeling overwhelmed by providing care. Social support can be protective for caregiving, but there is little research on the composition of social networks of caregivers. The research describes these social networks. Caregivers were recruited from a neuro-oncology clinic at an NCI-designated comprehensive cancer center in the United States from May to August 2017. Caregivers listed social network resources that they either have approached or could approach for help in six caregiving areas. Twenty-eight caregivers provided social network data. Most caregivers had at least some support in each category, with the most people in hands on and emotional support. Communication and financial support were less populated and were most likely to have no resources listed. Most resources were unique to one support category, but a smaller number of resources provided multiple types of support. Our data provide information for targeting interventions to address support needs in caregivers of patients with PMBT. These findings also represent the first time the compositions of the social networks of caregivers of patients with PMBT have been presented.