Primary mucinous cystadenocarcinoma (MCA) of the breast is a rare but pathologically distinct breast tumor. There have been some case reports on primary MCA of the breast; however, they have all focused on pathologic findings. Here, we report the radiologic findings of two cases of MCA along with a review of the literature. Breast MCA shows a circumscribed mass with some calcifications on mammography, an intracystic solid mass without increased vascularity or a vascular stalk on ultrasound, and a heterogeneously enhancing mass within a rim-enhancing cyst with intermediate signal intensity on T2-weighted magnetic resonance imaging. These radiologic findings and the presence of mucin in the percutaneous biopsy specimen should suggest the possibility of MCA in the differential diagnosis of a breast tumor.