Lymphoid leukemia of T-cell origin was diagnosed in a male Chinese Box turtle, Cuora flavomarginata, of approximately 25 years of age. The turtle presented with a history of anorexia, open-mouth breathing, and lethargy for one week. The CBC findings included a mildly increased PCV, and severe leukocytosis due to high numbers of atypical cells interpreted to be blasts. The blasts were medium-sized cells with round to pleomorphic nuclei, slightly clumped chromatin, indistinct nucleoli, and scant moderate-to-dark blue cytoplasm with occasional red-to-purple cytoplasmic granulation. Cytochemical and immunohistochemical staining indicated that the neoplastic cells were positive for CD3 and α-naphthyl butyrate esterase (ANBE), leading to the diagnosis of T-cell lymphoid leukemia. Histology of tissues collected at necropsy showed multifocal infiltrations of neoplastic round cells in the liver, spleen, kidneys, testicles, pancreas, thyroid, duodenum, bone marrow, epicardium, and myocardium. Transmission electron microscopy failed to identify viral particles within the neoplastic cells. This article describes the hematologic, histologic, and ultrastructural abnormalities associated with lymphoid leukemia in this turtle, and advanced diagnostic methods used for phenotyping the T-cell origin.