Alveolar soft part sarcoma (ASPS) is a rare neoplasm that most commonly presents in the lower extremities. Although ASPS has distinctive histologic features, it may cause diagnostic problems when it arises in unusual locations. To our knowledge, only 1 case of ASPS arising within the breast has previously been reported. Here, we report a second case of primary mammary ASPS. The patient was a 44-year-old woman who presented with a breast mass. Needle biopsy was performed, yielding a polygonal cell lesion with abundant, predominantly xanthomatous cytoplasm. The cells labeled strongly for the histiocytic marker CD68, suggesting a benign macrophage-rich lesion. However, the unusual nature of the lesion as well as the prominence of nucleoli prompted suggestion for an excision. The excision more clearly revealed the lesion's alveolar architecture and demonstrated cells with more eosinophilic cytoplasm, along with the xanthomatous cells. The diagnosis of ASPS was confirmed by electron microscopy, which revealed characteristic membrane-bound rhomboidal crystals, as well as by nuclear labeling for TFE3 protein by immunohistochemistry. With this report, we confirm the utility of a novel immunohistochmical technique for the identification of an ASPS presenting in an unusual locale.