BACKGROUND: Mobile health technologies have the potential to enhance the management, communication, and overall quality of life of patients with cancer. The MyPal project, using a participatory design approach, aims to provide palliative care support for patients with cancer through an electronic patient-reported outcomes eHealth platform. OBJECTIVE: This study aims to evaluate the usability and user experience of "MyPal for adults," a co-designed palliative care mobile app designed to support adults with cancer. METHODS: Representative users participated in a 4-step usability study using a "think-aloud" protocol, complemented with feature satisfaction, difficulty perceived, and design impression surveys along with a short, structured interview. Participants were also asked to provide quantitative feedback via the postuse System Usability Scale, User Experience Questionnaire, and Post-Study System Usability Questionnaire. The data were analyzed along the lines of the International Organization for Standardization (ISO) 9241-210 framework. RESULTS: All participants found the intervention content useful, and they reported satisfactory usability, with a mean Post-Study System Usability Questionnaire score of 2.458 (SD 1.08) and a System Usability Scale score of 68.9. All aspects of the User Experience Questionnaire (attractiveness, perspicuity, efficiency, dependability, stimulation, and novelty) surpassed usability quality benchmarks. The qualitative analysis identified 43 usability issues, primarily related to effectiveness and efficiency as defined in ISO 9241-210. CONCLUSIONS: In this study, we conducted a usability evaluation of the "MyPal for adults" app, a digital tool designed to enhance the palliative care experience. This approach identifies real-world usability issues, enabling iterative improvements in the eHealth platform's design. Our findings reveal a user-friendly interface and positive patient experiences. This study emphasizes the need to enhance mobile health platform usability, offering insights to improve digital palliative care.