Prostate cancers commonly metastasize to the pelvic lymph nodes and bones, but malignant pleural effusion rarely occurs. Prostate cancer lesions rarely cytologically mimic lymphomas. We present the case of a 70‑year‑old man with a history of chronic obstructive pulmonary disease and prostate cancer after hormone therapy, as well as a rare manifestation of pleural effusion. Chest radiography revealed a large amount of left pleural effusion. The cytological analysis of the pleural fluid purely consisted of single tumor cells, which were not immunoreactive to Nkx3.1 or prostate‑specific antigen. This was initially misinterpreted as a lymphoma. However, a further pleural biopsy revealed immunoreactivity to Nkx3.1, suggesting metastatic prostate cancer. This study showed that cytology and further biopsy effectively distinguished between Gleason 5 prostate cancer and lymphoma in a patient with pleural effusion.