Background: Globally, breast cancer has been identified as the most common cancer among women. The clinical efficacy of adjuvant oral antiestrogen therapy-including tamoxifen and aromatase inhibitors-has been proven to be clinically efficacious for breast cancer survivors. However, medication adherence for these therapies remains suboptimal among breast cancer survivors. Objective: The aim of this study was to evaluate the effect of a reminder intervention-a smart pill bottle paired with the Pillsy mobile application-on medication adherence, medication self-efficacy, and depression, among breast cancer survivors who were undergoing oral antiestrogen therapy. Methods: This study is a randomized controlled trial. Sixty-one women were allocated to an experimental group (n = 31) and the control group (n = 30). The experimental group received the reminder intervention of a smart pill bottle for 4 weeks. Study outcomes were identified as medication adherence, medication self-efficacy, and depression. Results: Fifty-seven women completed the follow-up measurement. Significant differences in favor of the experimental group were noted for medication adherence ( P = .004) and medication self-efficacy ( P = .004). There was no statistically significant difference between the 2 groups with regard to depression ( P = .057). Conclusions: Reminder intervention using smart pill bottles was effective in improving medication adherence and medication self-efficacy among breast cancer survivors undergoing oral antiestrogen therapy.