PURPOSE: To describe medication adherence, to analyze the relationships among medication adherence, illness perception, and beliefs about medicines, and to determine the mediating effects of beliefs about medicines on the relationship in breast cancer patients with adjuvant endocrine therapy (AET) in China. METHODS: A cross-sectional study was conducted on 202 breast cancer patients with AET from September 2017 to February 2019 in China. The Medication Adherence Report Scale (MARS-5), the Chinese version of the revised illness perception questionnaire for Breast Cancer (CIPQ-R-BC) and the Beliefs about Medicines Questionnaire (BMQ) were used. RESULTS: The mean MARS-5 score of our participants was 23.72 (SD = 1.62), and 175 (86.6%) patients were adherent to medications. Moreover, medication adherence was negatively correlated with identity, environmental or immune factors, emotional representations, BMQ-specific concerns, BMQ-general overuse, and BMQ-general harm, as well as being positively correlated with coherence and the total BMQ scores. Furthermore, beliefs in the overuse about medicines functioned as mediators for the influencing effects of coherence and emotional representations on medication adherence. CONCLUSION: Illness perception not only directly affected medication adherence, but also indirectly affected medication adherence through the beliefs about medicines. Necessary interventions that target beliefs in the overuse about medicines in breast cancer patients with AET with low levels of coherence or high levels of emotional representations could be provided to improve the level of their medication adherence.