Fibromatosis of the breast is a rare soft tissue lesion that arises from the mammary tissue or the pectoral fascia. We present a case of fibromatosis in a 39-year-old male patient who developed a right lateral breast mass in several weeks without prior trauma or surgery. Ultrasound-guided core needle biopsy findings included differential diagnoses of nodular fasciitis and fibromatosis. https://hollandscountryclothinges.shop/product-category/gun-blue/