Histologic and immunohistochemical features of a malignant mesothelioma of the testicular tunica vaginalis in a 9-year-old male mixed-breed dog are reported. The dog had a large scrotal swelling determined by ultrasonography to be a heterogeneous irregular mass characterized by a mixed echogenic pattern and located in the left vaginal canal of the testis. Histologically, the neoplasm was nodular, was poorly demarcated, and consisted of pleomorphic cells that were associated with a significant desmoplastic reaction. By immunohistochemistry, neoplastic cells stained positively for cytokeratin and vimentin and were immunoreactive with an antimesothelial cell monoclonal antibody. Malignant mesotheliomas of tunica vaginalis testis are rare in mammalian species, including dogs.