A Case of seminoma found in an adult guinea fowl (Numida meleagris) that has been exhibited in a zoo is reported. The right testis was extremely enlarged and replaced by round and polyhedral pleomorphic neoplastic cells showing nest, sheet, and diffuse growth patterns. The neoplastic cells had acidophilic cytoplasm and hyperchromatic and eccentrically placed nuclei. Metastatic lesions composed of diffuse growth of neoplastic cells similar to those of the primary tumor were seen in the liver, lungs, kidneys, and heart, and neoplastic emboli were often detected within blood vessels of these organs, indicating hematogenous metastasis. This is the first report of malignant seminoma with multiple metastases in the visceral organs in the guinea fowl.