Inflammatory myofibroblastic tumors (IMTs) are uncommon, mesenchymal lesions, and malignant transformation is extremely rare. The current study presents the case of a 56-year-old female with a rapidly growing mass in the right breast, which was diagnosed as IMT. Immunohistochemically, the mass was positive for smooth muscle actin (SMA) and Ki-67 (positive staining in 30% of the cells), and negative for S-100, cluster of differentiation (CD)34, p63 and cytokeratin. Malignant transformation to metaplastic carcinoma of the spindle-cell type was observed following surgical resection. This metaplastic carcinoma demonstrated positive immunoreactivity for cytokeratin, vimentin, CD34, p63 and Ki-67 (>30%), and was negative for cytokeratin 7, SMA, desmin and S-100. The patient underwent total mastectomy of the right breast, followed by palliative chemotherapy with capecitabine; however, the patient succumbed to the disease after 12 weeks. The unusual case presented in the current study emphasizes the importance of pre-operative examinations to determine the benign or malignant nature of IMTs, which aids in the choice of appropriate surgical procedures.