A 22-year-old Australian stockhorse gelding was presented with anterior uveitis in the right eye which was nonresponsive to anti-inflammatory therapy. Clinical examination revealed corneal edema and vascularization, marked hypopyon, and thickening of the dorsal iris, which was confirmed by ultrasonography. Hematologic and biochemical analyses, abdominal and thoracic ultrasonography, and abdominocentesis with cytologic and biochemical analysis revealed no significant abnormalities. Cytological examination of an aqueous humor sample revealed a population of predominantly large lymphoblasts with high nuclear-to-cytoplasmic ratio, round or irregular nuclei, clumped nuclear chromatin, multiple large prominent nucleoli, and a small volume of basophilic cytoplasm. The cytologic diagnosis was intraocular lymphoma. Biopsy of the right submandibular lymph node revealed no evidence of neoplastic invasion. Euthanasia and a complete necropsy were performed and revealed no evidence of neoplasia in any tissue other than the right eye, which had an extensive, well-defined infiltrate of neoplastic lymphocytes expanding the ciliary body and iris, infiltrating the ciliary epithelium, and extending into the pars plana and peripheral choroid. Immunohistochemistry confirmed that neoplastic cells expressed the T-cell marker CD3. To the authors' knowledge, this is the first description of primary, solitary uveal T-cell lymphoma in a horse. Although apparently rare, lymphoma should be considered in horses with uveitis, even when inflammation is unilateral and in the absence of extraocular signs of neoplasia. Aqueocentesis and cytological examination provided an antemortem diagnosis in this case and should be considered as a diagnostic tool for investigation of uveal thickening and hypopyon.