A 14-year-old female South American sea lion (Otaria byronia) with persistent vaginal secretion and chronic hemorrhagic diarrhea was encountered. During postmortem examination, the uterus was found to resemble a balloon with mucosal congestion and was filled with grayish milky material. The ovaries also had abnormal features, including necrotic surface lesions and multiple whitish foci in the cut section. Hemorrhages and ulcerated changes due to toxemia were observed in other organs, including the liver, spleen, lung, intestines, and lymph nodes. Microscopically, the left ovary contained interlacing fascicles of fibroblast-like cells with blunt-end nuclei showing cytoplasmic positive immunoreactivity against alpha-smooth muscle actin and desmin. The right ovary contained cells with round to cigar-shaped nuclei showing cytoplasmic positive immunoreactivity against vimentin. In conclusion, based on classification of bilateral ovarian tumors as a leiomyoma in the left region and a fibroma in the right region, this sea lion was diagnosed with chronic closed pyometra.