Background: Mobile applications are playing an increasing role in patient care, and their use in the oncologic field opens up promising possibilities in the fields of supportive cancer care and patient education. The objective of our study was to determine the effects on quality of life (QOL) of a mobile app–based training for supportive care of women with breast cancer using adjuvant endocrine hormonal therapy. Material and Methods: A single-blinded and randomized design was used. Participants were randomly assigned to either control group that receives routine care or intervention group that receives routine care plus access to mobile app–based training support for 12 weeks. QOL and symptom distress were measured before intervention (T0), after 12 weeks (T1) of intervention. This study is the application of three modality combinations: mobile app-based patient education (1), web-based management application (2), and nurse’s tracking over the phone and mobile app (3). The mobile app-based training also provides basic information about breast cancer, symptom diary and lifestyle recommendations (adequate and balanced nutrition, regular physical activity, deal with stress effectively). Results: QOL of the treatment group after intervention increased and distress level was low compared to the control group; these results were statistically significant. The majority of the patients reported that the mobile application was “informative and useful”. Conclusions: This demonstrated that the mobile app is an effective intervention for supportive care in women with breast cancer. The mobile app–based training, which is an innovative intervention, is recommended as a supportive care initiative for women with breast cancer.