A well-circumscribed mass 70 x 35 x 28 cm in size and 41 kg in weight was detected at necropsy in a male adult horse within the omentum major without any association to the gastrointestinal tract. The tumor consisted of multiple white-to-yellow lobules and displayed a firm consistency. In addition, multiple cysts filled with blood-like fluid, and multifocal areas of necrosis were observed. Histologically, the mass consisted of slightly pleomorphic spindloid-shaped cells arranged in interlacing bundles containing elongated nuclei with blunt ends. The majority of tumor cells revealed a positive immunoreaction for alpha-smooth muscle actin, vimentin, and neuron-specific enolase and were negative for S-100, factor VIII-related antigen, and glial fibrillary acidic protein. Few tumor cells showed expression of desmin and c-kit. On the basis of macroscopy, histology, and immunohistochemistry, an omental leiomyoma was diagnosed.