Granular cell tumors (GCTs) are uncommon soft tissue tumors that mostly occur in patients between 40 and 60 years of age and can occur at various body sites. Malignant granular cell tumors (MGCTs) comprise less than 2 % of GCTs and are mostly found on the lower extremities, especially the thighs. These tumors grow more rapidly than benign GCTs, and most importantly, they can metastasize. We describe an MGCT that presented as a right breast mass in a 79-year-old Japanese woman. Local excision was performed for the primary tumor, which was diagnosed as an atypical GCT, but 15 months later, the tumor recurred at the same site. Thereafter, right mastectomy with axillary lymph node dissection was performed. Metastatic disease was identified in 2 of 12 lymph nodes. The pathological examination revealed that the tumor had progressed to an MGCT after recurrence. Multiple liver, lung and bone metastases were revealed 4 months after the second surgery, and the patient died 34 months after the primary surgery. Our findings highlighted the difficulty in diagnosing MGCTs using histological features alone and suggested the usefulness of Ki67 values. A tumor with a high level of Ki67 should be treated as malignant, even if the tumor has few pathological features of malignancy.