A 19-yr-old female Linnaeus's two-toed sloth (Choloepus didactylus) with a history of urinary incontinence, ascites, and behavioral changes was euthanized after diagnostic imaging revealed a large bladder mass. On gross necropsy, the sloth had a severely thickened bladder mucosa, partial urinary obstruction, and nonseptic exudate in the peritoneal cavity. Histopathology showed a malignant and highly invasive transitional cell (urothelial) carcinoma with transmural and intra-abdominal invasion and diffuse carcinomatosis. Immunohistochemistry for expression of pancytokeratin (AE1/AE3), cytokeratin 7 (CK7), cytokeratin 20 (CK20), and uroplakin III was performed to confirm the diagnosis of transitional cell carcinoma. Neoplastic cells had a strong cytoplasmic immunoreactivity with the antipancytokeratin antibody clone AE1/AE3, which was consistent with a neoplasm of epithelial origin. Neoplastic cells were negative for expression of CK20. This is the first detailed report describing the antemortem diagnosis of urinary bladder transitional cell carcinoma with carcinomatosis in a two-toed sloth.