A case of an ovarian tumor is reported in an 8-yr-old South American sea lion (Otaria flavescens) kept in a marine park in Malta (35.57 degrees N, 14.25 degrees E). The neoplasm was a solid mass of dense sheets and nests of round to polyhedral, irregularly shaped cells with abundant, finely vacuolated cytoplasm. The nuclei were uniformly small and round to oval. The supporting stroma contained thecal cells. The tumor cells were positive for positive inhibin and vimentin and focally positive for cytokeratin by immunohistochemistry. The neoplasm was diagnosed as an ovarian sex cord-stromal tumor, specifically an interstitial cell tumor.