Clear cell adenofibroma (CCAF) is a rare surface epithelial-stromal tumor of the ovary and recently considered another precursor of clear cell adenocarcinoma (CCA) other than endometrioma. We report magnetic resonance (MR) findings of a borderline CCAF that contained a small CCA focus. The tumor manifested a characteristic "black sponge" appearance. The CCA focus showed high signal intensity on diffusion-weighted imaging (DWI) and early enhancement on dynamic contrast-enhanced (DCE) MR imaging (DCE-MRI), and the CCAF components showed low signal intensity on DWI and gradually increasing contrast enhancement on DCE-MRI.