Background <p>Mammary metaplastic squamous cell carcinoma is an exceptionally rare and aggressive triple-negative breast cancer. Its propensity to mimic benign breast abscesses poses a significant diagnostic challenge, often leading to treatment delays.</p> Case presentation <p>A 41-year-old Asian woman presented with a rapidly enlarging right breast mass, initially misdiagnosed and treated as an abscess at a local hospital. Definitive biopsy revealed triple-negative mammary metaplastic squamous cell carcinoma. The patient received neoadjuvant chemotherapy with a nab-paclitaxel, epirubicin, cyclophosphamide regimen. After an initial partial response observed in the first five cycles, the tumor exhibited progressive disease following the sixth cycle, a phenomenon potentially indicative of acquired chemoresistance. The patient subsequently underwent a modified radical mastectomy. Given the high-risk features and inspired by the SYSUCC-001 trial, adjuvant dose-dense capecitabine was administered postoperatively. No recurrence was detected during follow-up.</p> Conclusion <p>This case highlights a classic diagnostic pitfall of mammary metaplastic squamous cell carcinoma masquerading as an abscess and illustrates a unique pattern of acquired resistance to a standard taxane-anthracycline neoadjuvant chemotherapy regimen. It underscores the need for heightened clinical vigilance. The potential benefit of adjuvant capecitabine for patients with high-risk mammary metaplastic squamous cell carcinoma observed here warrants further investigation.</p>