A 6-yr-old male Sumatran tiger (Panthera tigris) with no significant past clinical history was anesthetized for clinical examination after 5 days of lethargy. Clinically, the animal presented with anorexia, pale mucous membranes, and icterus. Hematologic results indicated moderate anemia and severe thrombocytopenia and showed a circulating population of atypical mast cells. The tiger died during anesthesia. On postmortem examination, abdominal hemorrhage associated with marked diffuse hepato-splenomegaly and mesenteric, hepatic, and splenic lymph node hypertrophy were observed. A visceral mast cell tumor was confirmed by histologic examination and toluidine blue staining, with splenic, hepatic, lymphoid, renal, and pulmonary infiltration. Hematologic, postmortem, and histologic findings were consistent with mastocytemia associated with the splenic form of mast cell tumor described in domestic cats.