Introduction: In recent years, the prevalence of breast cancer has increased globally, including in Indonesia. Despite its high prevalence, patient adherence to treatment remains low. Social support is known to be one of the factors that can increase patient adherence. Although many studies have explained the relationship between social support and adherence in breast cancer patients, the underlying mechanism remains unclear. The aim of this study was to examine the relationship between social support and patient adherence using uncertainty as a mediator. Methods: A descriptive and correlational survey was conducted in a regional hospital in Surabaya, Indonesia. A convenience sample of 165 Indonesian breast cancer patients voluntarily participated in the study and responded to a set of questionnaires, including a socio-demographic questionnaire, the Social Support Scale (SSS), the ShortForm Mishel Uncertainty in Illness Scale (SF-MUIS), and the Morisky Medication Adherence Scale (MMAS8). Results: The results revealed that breast cancer patients with higher social support found it easier to undergo treatment, resulting in better adherence. Patients who experienced higher social support had reduced uncertainty through improved subjective perceptions and reduced evaluations of the severity of stressful illness events. In addition, uncertainty significantly mediated the relationship between social support and patient adherence. Conclusions: These findings indicate that social support for breast cancer patients improves patient adherence. Moreover, healthcare providers can help breast cancer patients