Purpose <p>This study aimed to explore potential prognostic associations between adjuvant chemotherapy and survival outcomes in patients with T1b-T2N0M0 medullary breast cancer (MBC) who underwent surgical resection (breast-conserving surgery or mastectomy).</p> Methods <p>Using the Surveillance, Epidemiology, and End Results (SEER) database (2000–2015), we analyzed 2343 women with pathologically confirmed T1b-T2N0M0 MBC who underwent surgery. Patient characteristics, treatment (chemotherapy, radiotherapy, surgery type), and survival outcomes (overall survival [OS], breast cancer-specific survival [BCSS]) were analyzed. Propensity score matching (1:1) was employed to minimize confounding. Kaplan–Meier analysis and Cox proportional hazards models assessed survival differences and prognostic factors.</p> Results <p>Among 2343 patients (median follow-up 141&#xa0;months), 1507 (64.3%) received adjuvant chemotherapy. Patients receiving chemotherapy were typically younger (&lt;65&#xa0;years), more likely to be married, had larger tumors (T1c/T2), and had estrogen receptor-negative disease than those who did not. Multivariable analysis identified adjuvant chemotherapy as an independent favorable prognostic factor for both OS (<i>P</i> &lt; 0.001) and BCSS (<i>P</i> = 0.039). Before and after propensity score matching, chemotherapy significantly improved OS and BCSS (<i>P</i> &lt; 0.05). Subgroup analysis revealed significant survival benefits for patients with T1c-T2 tumors and hormone receptor-negative/human epidermal growth factor receptor 2-negative disease. However, no significant benefit was observed for patients with T1b tumors or hormone receptor-positive/human epidermal growth factor receptor 2-negative disease (<i>P</i> &gt; 0.05).</p> Conclusions <p>Adjuvant chemotherapy may optimize survival in select patients with T1b-T2N0M0 MBC, though prospective validation is warranted.</p>