Background: Metaplastic carcinoma of the breast is a rare but distinct entity within the group of undifferentiated invasive carcinomas. This entity accounts for less than 0.5% of all breast cancers and contains elements of epithelial (ductal), mesenchymal, and intermediate forms of differentiation. Of these metaplastic carcinomas, there have been reports of chondroid, squamous, osseous, and spindle differentiation. Case Report: We describe the clinical course of a 52-year-old female patient with an unusual histopathology of a spindle cell carcinoma of the breast, discuss the literature, and recommend an approach to diagnosis and treatment. The patient initially presented to an outside hospital with a rapidly growing breast mass that was originally diagnosed as a malignant phylloides tumor. She presented 11 months after the primary treatment with a local subcutaneous relapse. She later developed a local relapse of her metaplastic carcinoma in the chest wall. Extensive histopathological analysis lead to the diagnosis of a spindle cell metaplastic breast carcinoma with leiomyoid differentiation, which has not been described previously. Conclusions: Overlap in morphology can lead to a misinterpretation or underdiagnosis of metaplastic carcinomas. However, the prognosis is similar to more common types of breast adenocarcinoma.