A 20-year old male cotton-top tamarin (Saguinus oedipus) was presented with unilateral enlargement of an intrascrotal testicle. Fine-needle aspiration cytology demonstrated a neoplastic population with Call-Exner-like bodies and features of malignancy. The animal was castrated, and histologic examination revealed a biphasic sex cord-stromal tumor, with one region resembling Sertoli-cell tumor and one region resembling granulosa-cell tumor, with extensive microfollicular pattern and many Call-Exner bodies. Eight months after castration, the animal was euthanized on discovery of a caudal abdominal mass that displaced organs, was highly infiltrative, and extended into the paravertebral musculature with lysis of vertebral bone. Metastases to lymph node and lung were also present. Histologic examination of the abdominal tumor showed multifocal formation of Call-Exner bodies in an otherwise highly dedifferentiated population. Positive immunolabeling for alpha inhibin confirmed the sex cord-stromal origin of the abdominal and paravertebral tumor masses. This case has similarities to malignant testicular granulosa-cell tumor of humans.