Background: Patient engagement is critical for realizing the value of telehealth modalities such as the patient portal. Family caregiver engagement may also be critical for facilitating the use of the patient portal among adult patients, including older adults. Objective: This study aims to analyze the 2019 Health Information National Trends Survey to characterize family caregivers’ use of their care recipient’s patient portal in terms of sociodemographic, health, and caregiving characteristics and caregivers’ use of their own portal. Methods: We conducted a secondary analysis of cross-sectional data from the National Cancer Institute’s Health Information National Trends Survey 5 Cycle 3. This survey was administered to 5438 US adults between January and May 2019. We analyzed data from 320 respondents who were identified as family caregivers. We created measures to reflect family caregivers’ use of their care recipient’s and their own portal, caregiver demographic and caregiving characteristics, and care recipient health characteristics. Results: Over half of the caregivers (179/320, 55.9%) reported using their own portal at least once, whereas only one-third (105/320, 32.8%) reported using their care recipient’s record in the previous 12 months. Caregivers using their own portal were significantly more likely to use their care recipient’s portal (odds ratio 11.18; P<.001). Conclusions: Policies should enable patients to designate family caregivers who can access their patient portal. Providers could screen caregivers for challenges in their caregiving responsibilities that may be addressed through the portal so they can better support their adult relatives. Interventions to support family caregivers, especially older caregivers, in using their own portal may facilitate their use of their care recipient’s portal.