A 59-year-old man was admitted with a large amount of ascites, cake-like omental thickening, and dialysis-associated acquired cystic disease of the kidney (ACDK). It was difficult to detect renal cancer, which was revealed as a primary site of peritoneal metastases by autopsy, with conventional cross-sectional imaging, such as enhanced computed tomography and T2, T1, and dynamic gadolinium-enhanced T1-weighted magnetic resonance imaging, because multiple renal cysts caused marked distortion of the renal parenchyma and the cancer had necrosis. We demonstrated the usefulness of diffusion-weighted imaging with a high b-factor to detect renal cancer presenting with peritoneal metastasis in a patient with ACDK.