Desmoid-type fibromatosis (DTF) is a rare and locally aggressive benign tumor affecting the breasts. It is characterized by infiltrative growth and local recurrence, though it cannot metastasize. We present the case of a woman in her early 20s diagnosed with a palpable mass within her right breast. Imaging evidence was classified as Breast Imaging Reporting and Data System (BI-RADS) category 4 and above. We performed ultrasound-guided core needle biopsy, and pathological examination revealed hyperplasia of ductal epithelium and lobular tissue. Imaging results suggested a greater likelihood of the mass being malignant; however, pathological outcomes indicated that it was benign, resulting in the decision to perform an extended mastectomy. Postoperative pathological results suggested DTF. However, no further treatment was performed after surgery, and the patient's breast color Doppler ultrasound evaluation 12 months after surgery showed no recurrence or metastasis.