An intact male white pekin duck (Anas platyrhynchos domesticus) was presented for examination because of respiratory distress and 2- to 3-month history of lameness and lethargy. Results of radiography, ultrasonography, and cytologic examination revealed a large neoplastic mass in the coelom. The duck was euthanatized, and results of necropsy revealed 2 large, lobulated masses in the coelom and a small nodule on the liver. Histopathologic examination of the large masses revealed a collision pattern testicular tumor consisting of Sertoli, seminoma, and interstitial cell components. The hepatic nodule was a metastatic lesion consistent with a Sertoli cell testicular tumor. This is the first reported case of a mixed germ cell–sex cord–stromal tumor in a duck.