An adult Goldfish (Carassius auratus) developed an exophitic, multinodular, mass in the dorsal region involving the fin. The clinical and pathologic approach included ultrasonography, cytology, histopathology, immunohistochemistry, and ultrastructural evaluation. B-mode and color Doppler ultrasonographic findings showed an oval, well-defined mass with slightly inhomogeneous parenchymatous echotexture associated with an intense intralesional vascularization. Cytology and histology revealed neoplastic pleomorphic spindle cells arranged in a storiform pattern or in palisades typical of Antoni A pattern schwannoma. Moderate anisocytosis and anisokaryosis and occasional binucleation were also present. The neoplastic tissue deeply infiltrated the skeletal muscle of the dorsal region. Immunohistochemistry showed a diffuse cytoplasmic immunoreactivity of neoplastic cells to S100 protein and proliferating cell nuclear antigen (PCNA); glial fibrillary acidic protein was negative. The PCNA proliferation rate was 23.5% (calculated as the mean of 10 fields). Ultrastructurally, neoplastic cells were juxtaposed with parallel nuclei forming the typical palisade pattern and sharing cytoplasmic and nuclear features with human schwannoma. This is the first cytologic description supplied with echographic investigation of a malignant schwannoma in a fish species.