Parenchymal leiomyomas of the breast are extremely rare, with only 25 cases reported in the literature. We report the 26th case of parenchymal leiomyoma of the breast in a 63-year-old woman who presented to us with a right breast tumor detected on her screening mammography. This tumor was evaluated by mammography, ultrasonography, and MR imaging. To the best of our knowledge, this is the first case of a parenchymal leiomyoma of the breast examined by MR imaging. MR imaging revealed an oval mass with circumscribed margins that appeared as a high-intensity lesion in both T1 and T2. A dynamic MRI study showed a gradual increase pattern on the mass. Excisional biopsy revealed a growth pattern of interlacing fascicles of spindle cells without atypia or mitoses, consistent with parenchymal leiomyoma of the breast. Here we report a case of parenchymal leiomyoma of the breast, and describe the clinical, pathological, and immunohistochemical findings. In addition, we review the literature on parenchymal leiomyoma of the breast with regard to clinical characteristics and pathological features of this entity.