Primary tumors of serosal surfaces are uncommon in veterinary medicine. Mesothelial neoplasms can be benign or malignant, and are classified as predominantly epitheloid, mixed (biphasic), or fibrous (spindle cell, fibrosarcomatous), with fibrous mesotheliomas reported least in domestic species. A 9-year-old Domestic Shorthair cat presented on emergency with a brief history of weakness and lethargy. On presentation, the cat was semicomatose, hypothermic, and hypotensive with a markedly distended abdomen. Approximately 1 liter of serosanguineous fluid was removed via abdominocentesis. Diagnostic imaging and cytologic evaluation of fine-needle aspirates were suggestive of neoplasia, and the cat was subsequently euthanized. At necropsy, the omentum was contracted cranially into an irregular lobular mass that surrounded the stomach and proximal intestinal tract, and focally infiltrated the spleen. Both visceral and parietal peritoneal surfaces were thickened and contained off-white friable material and occasionally firm fibrous plaques. Microscopically, serosal surfaces were expanded by neoplastic spindle cells, which were often accompanied by moderate to abundant fibrous stroma. Neoplastic cells had varying degrees of immunoreactivity for cytokeratin, vimentin, desmin, and smooth muscle actin, which was consistent with the diagnosis of mesothelioma.