Microscopically, ovarian malignant Brenner tumor shows components of malignant transitional cell tumor intermixed with benign and borderline elements. We report a case of this tumor with components that transition from benign to malignant on diffusion-weighted images (DWI) and in histologic findings. The benign component showed low signal intensity on T(2)-weighted images, whereas the malignant component showed high signal intensity on T(2)-weighted images and high signal intensity on DWI with low apparent diffusion coefficient (ADC) value.