Background <p>Self-advocacy is a key indicator of effective patient participation in disease treatment and an effective way to promote patients' self-management skills, which plays an important role in improving patients' quality of life. However, we know little about the mechanisms of self-efficacy and self-advocacy in young and middle-aged breast cancer patients.</p> Aim <p>To explore the mediating role of health literacy between self-efficacy and self-advocacy in young and middle-aged breast cancer patients, and to provide a theoretical basis for the implementation of intervention strategies to enhance self-advocacy in breast cancer patients.</p> Methods <p>This study utilized a cross-sectional research design and included 262 young and middle-aged breast cancer patients as study participants. These patients completed paper-based questionnaires covering demographic information, self-efficacy assessment, health literacy measures, and a self-advocacy survey. SPSS 26.0 software and AMOS 24.0 software were used to analyze the data, and structural equation modeling was constructed to provide insight into the relationships among the variables.</p> Results <p>The results of the study showed that the self-advocacy score was 80.94 ± 8.56. Self-advocacy was positively correlated with self-efficacy (<i>r</i> = 0.305;<i> p</i> &lt; 0.01) and health literacy (<i>r</i> = 0.484; <i>p</i> &lt; 0.01), while self-efficacy was positively correlated with health literacy (<i>r</i> = 0.327; <i>p</i> &lt; 0.01). In addition, health literacy partially mediated the effect between self-efficacy and self-advocacy (mediating effect of 0.254,95% CI: 0.147–0.398, accounting for 54.3% of the total effect).</p> Conclusions <p>Self-efficacy of young and middle-aged breast cancer patients has a significant direct impact on self-advocacy and further enhances self-advocacy through health literacy. Therefore, enhancing patients' self-efficacy and improving health literacy are effective ways to improve self-advocacy.</p>