Approximately 6.1 million adults in the United States serve as care partners for cancer survivors. Studies have demonstrated that engaging cancer survivors and their care partners through technology-enabled structured symptom collection has several benefits. Given the high utilization of mobile technologies, even among underserved populations and in low resource areas, mobile apps may provide a meaningful access point for all stakeholders for symptom management. We aimed to develop a mobile app incorporating user preferences to enable cancer survivors' care partners to monitor the survivors' health and to provide care partner resources. An iterative information gathering process was conducted that included (1) discussions with 138 stakeholders to identify challenges and gaps in survivor home care; (2) semistructured interviews with clinicians (n=3), cancer survivors (n=3), and care partners (n=3) to identify specific needs; and (3) a 28-day feasibility field test with seven care partners. Health professionals noted the importance of identifying early symptoms of adverse events. Survivors requested modules on medication, diet, self-care, reminders, and a version in Spanish. Care partners preferred to focus primarily on the patient's health and not their own. The app was developed incorporating quality-of-life surveys and symptom reporting, as well as resources on home survivor care. Early user testing demonstrated ease of use and app feasibility. TOGETHERCare, a novel mobile app, was developed with user input to track the care partner's health and report on survivor symptoms during home care. The following two clinical benefits emerged: (1) reduced anxiety among care partners who use the app and (2) the potential for identifying survivor symptoms noted by the care partner, which might prevent adverse events. ClinicalTrials.gov NCT04018677; https://clinicaltrials.gov/ct2/show/NCT04018677. ©Ingrid Oakley-Girvan, Sharon Watkins Davis, Allison Kurian, Lisa G Rosas, Jena Daniels, Oxana Gronskaya Palesh, Rachel J Mesia, Arif H Kamal, Michelle Longmire, Vasu Divi. Originally published in JMIR Formative Research (https://formative.jmir.org), 13.08.2021.