We identified a malignant lymphoma infiltrating the lung, liver, kidney, mesenteric lymph nodes, and eye as the cause of death in a male West Indian manatee (Trichechus manatus). Diagnosis was based on gross, histopathologic, and immunohistochemical studies. Tissue samples from ten organs were included in a tissue microarray and sections from this array were subjected to immunohistochemical staining. The cytoplasm of the neoplastic lymphocytes identified in six organs was positive for CD3, a marker for T-cell differentiation. The neoplastic cells were negative for CD79a, a marker for B-cell differentiation. The cause of this neoplasm was not determined. This is the first report of malignant lymphoma in the mammal order Sirenia.