Cancer self-management interventions improve symptom management and confidence, but few interventions target the complex needs of older adults with cancer and multi-morbidities. Despite growing evidence of digital health tools in cancer care, many such tools have not been co-designed with older adults to ensure that they are tailored to their specific needs. The objective of the study was to design a self-and symptom-management app to support older adults with cancer and multi-morbidities. Utilizing a user-centered design thinking framework, we recruited 2 caregivers and 18 older adults with lived experiences of cancer to design a medium-fidelity app prototype. Participants highlighted the importance of tracking functions to make sense of the information about their symptoms, clear displays, and reminders to mitigate concerns related to polypharmacy. This app will create a ‘home base’ for symprtom management and support for older adults with cancer and multi-morbidities.