The relatively rare entity pyothorax-associated lymphoma (PAL) is an aggressive disease with a poor prognosis. Therefore, PAL should be diagnosed as soon as possible with minimal invasion. We herein report the case of an 81-year-old man with PAL that was successfully treated with chemotherapy. A computed tomography-guided tumor biopsy could not provide adequate specimens for pathological investigation, whereas a diffusion-weighted magnetic resonance imaging-guided tumor biopsy led to a diagnosis. Our findings suggest that diffusion-weighted image guidance for planning the biopsy of an intrathoracic mass, especially in cases of suspected PAL, is a highly accurate method because it provides information about cellularity and does not create calcification-related artifacts.