A case of unilateral seminoma with visceral metastases in a Mallard duck (Anas platyrhynchos) is reported. The right testis was markedly enlarged. The liver surface showed multifocal to coalescent regular circular umbilicated greyish-white spots. In addition, multiple rough whitish nodules were evident on the pancreas and the visceral peritoneum lining the intestine. Histologically, the right testicular parenchyma was diffusely affected and replaced by neoplastic growth, consisting of sheets of large round to polyhedral cells with conspicuous vesicular nuclei having distinctly granular chromatin and prominent nucleoli. Sheets of cells with similar features were observed also in the other affected organs. Multiple lung metastases were detected on histology. This is the first known report of seminoma with hepatic, pancreatic, pulmonary and peritoneal metastases in a Mallard duck.