A 12-year-old, female mangrove snake (Boiga dendrophila melanota) was presented with a 1-month history of anorexia, lethargy, and progressive distention of the caudal coelomic cavity. Based on ultrasonographic and cytological examination, an ovarian neoplasia was considered to be the presumptive antemortem diagnosis. Histological examination confirmed the diagnosis of a granulosa cell tumor (GCT) with metastatic tumors in the coelomic fat bodies. Four months following oophorectomy and surgical removal of the distant metastasis, the snake was euthanized because of sudden onset of lethargy, cardiomegaly, and dyspnea. Postmortem examination revealed the presence of metastatic tumors in the lung, liver, and kidneys. Primary ovarian neoplasms are relatively rare in reptiles. Although GCTs have been documented in snakes, definitive diagnosis is typically determined through postmortem histopathological examination. The present case describes a suitable antemortem diagnostic approach of a malignant GCT in a mangrove snake with distant metastases.