Informal caregivers are a critical source of support for cancer patients. However, their perspectives are not routinely collected, despite health impacts related to the burden of caregiving. We created the TOGETHERCare smartphone application (app) to collect observer-reported outcomes regarding the cancer patient’s health and caregiver’s perceptions of their own mental and physical health, and to provide tips and resources for self-care and patient care. We enrolled 54 caregivers between October 2020 and March 2021 from Kaiser Permanente Northern California (KPNC), an integrated healthcare system. Fifty caregivers used the app for approximately 28 days. Usability and acceptability were assessed using questions from the Mobile App Rating Scale (MARS), the System Usability Scale (SUS), the Net Promoter Score (NPS), and semi-structured interviews. The caregivers’ mean age was 54.4 years, 38% were female and 36% were non-White. The SUS total mean score was 83.4 (SD = 14.2), for a percentile rank of 90-95 (“excellent”). Median MARS responses to the functionality questions were also high. The NPS score of 30 at the end of the study indicated that most caregivers would recommend the app. Themes from semi-structured interviews were consistent across the study period and indicated that the app was easy to use and helpful. Caregivers indicated a need for feedback from the app, suggested some changes to the wording of questions, the app’s visuals, and timing of notifications. This study demonstrated that caregivers are willing to complete frequent surveys about themselves and their patients. The app is unique because it provides a remote method to collect caregivers’ observations about the patient that may be useful for clinical care. To our knowledge, TOGETHERCare is the first mobile app developed specifically to capture adult cancer patient symptoms from the informal caregiver’s perspective. Future research will examine whether use of this app can help improve patient outcomes.