Mucocele-like tumors (MLTs) of the breast are rare, with only 11 cases reported from Japan and 35 cases from other countries. MLTs of the breast were first described by Rosen in 1986. They are believed to be related to atypical ductal hyperplasia, ductal carcinoma, or mucinous carcinoma. It is difficult to diagnose this tumor preoperatively, and especially difficult to differentiate between benign and malignant forms. We report a case of MLT associated with ductal carcinoma in situ, which was initially diagnosed as fibroadenoma by mammography and ultrasonography, and as mucinous carcinoma by fine-needle aspiration cytology. We discuss the characteristic findings of imaging and the appropriate clinical treatment of this tumor. The characteristic image first signals the possibility of this tumor, following which the diagnosis can be confirmed by pathological examination of a fully excised tumor specimen. Breast-conserving surgery is recommended because of the low risk of high-grade malignancy, even when malignancy is confirmed, and lymph node dissection may be avoided.