PgRMC1 (progesterone receptor membrane component 1) is a progesterone-binding protein highly expressed in various cancer cells. Its expression has been associated with cancer development, chemoresistance, and poor prognosis in breast cancer through the regulation of drug and sterol metabolism, cell cycle, and apoptosis. However, the precise functions of PgRMC1 remain unclear. We examined PgRMC1 expression in breast cancer tissues and analyzed its correlation with clinicopathological characteristics and treatment response (n=112). PgRMC1 expression was detected in breast cancer tissue, while no immunoreactivity for PgRMC1 was observed in normal breast tissue. Notably, PgRMC1 expression levels were high in the HER2 subtype. Among patients treated with neoadjuvant chemotherapy, those with a poor pathological response had significantly higher PgRMC1 expression than those with a good response. PgRMC1 overexpression in breast cancer cell lines reduced chemosensitivity. Real-time qPCR analyses revealed that PgRMC1 induced the expression of differentiation markers, including CDH1 and KRT19, indicating that PgRMC1 is involved in the suppression of epithelial-mesenchymal transition and mammary luminal epithelial differentiation. Our findings suggest that PgRMC1 may contribute to chemoresistance and serve as a putative biomarker for assessing neoadjuvant chemotherapy sensitivity.