This study aimed to examine the effectiveness of a web application on health literacy and self-efficacy in managing arm oedema symptoms among patients with breast cancer. The research was carried out in four phases as follows: Phase 1, using a qualitative approach to explore problems and information needs in educating breast cancer patients through in-depth interviews with 10 professional nurses who had experiences in caring for breast cancer patients and 20 breast cancer patients. Data were analysed by content analysis; Phase 2, designing and developing a web application and confirming its quality by five experts with experience caring for breast cancer patients; Phase 3, testing the web application with five breast cancer patients; and Phase 4, examining the effectiveness of a web application in breast cancer patients using a quasiexperimental research method. Patients were divided into 15 persons in control and 15 in intervention groups, a total of 30 persons. The tools used in the study consisted of (1) a web application on practices for reducing risk for arm oedema after breast cancer treatment, (2) a health literacy assessment tool, (3) a self-efficacy for managing symptoms questionnaire and (4) a web application satisfaction questionnaire. Data were analysed using descriptive statistics, chi-square and t test. Qualitative findings: The web application should cover patients' and nurses' views on arm oedema causes, assessment, prevention and self-care for managing swelling after breast cancer treatment. Characteristics of a web application required: large text, bright colours, clear visibility, accompanying pictures or videos, using simple language without official terminology, easy to access, convenient to use, concise, interesting content and shareable to others. Quantitative findings: The intervention group had significantly higher health literacy and self-efficacy in managing symptom scores than before the trial (p < 0.001). Sample groups were satisfied with the developed web application at a high level. When considering each item, it was found that all items were rated at high levels. Two items with the same highest score were ease of use and the attractiveness of the presentation style. This web application, aimed at reducing the risk of arm oedema after breast cancer treatment, is an effective tool for educating all hospitalized patients. In addition, further research should be conducted to monitor the sustainability of long-term and clinical outcomes.