Patient-reported outcomes are increasingly emphasized in clinical trials and population health studies. Our research team developed a smartphone app to track patient-reported outcomes of children with chronic diseases. The purpose of this study is to develop a patient-reported outcome reporting app and evaluate its usability. A multidisciplinary research team including health services researchers, pediatric nurses, and software engineers worked collaboratively in developing the patient-reported outcome app and administration portal. Group discussions and several rounds of feedback and modification were used. Ten pediatric patients with cancer, five parents, and two nurses participated in the usability study. We conducted content analyses in app development and usability evaluation. The app collected demographic information and patient-reported outcomes. Patient-reported outcomes were collected by Chinese versions of pediatric Patient-Reported Outcomes Measurement Information System Short Forms and Patient-Reported Outcomes Measurement Information System Parent Proxy Report Scales for Children. Pediatric patients aged 8 to 17 years and parents with a 5- to 7-year-old pediatric child used different age-appropriate questionnaires. The Web-based administration portal helped to manage demographic information, questionnaires, administrators, and survey-conducting organizations. The users liked the app. All participants felt that this app was easy to use and the interfaces were friendly to children. Nurses thought the administration portal interfaces were simple and the data were convenient to download for further analysis. We conclude that the app and its administration portal meet researchers and clinical nurses’ demand and have potential to promote patient-reported outcomes in assessing quality of life and symptoms of pediatric patients.