The NOD.Cg-Prkdc^scid Il2rg^tm1Wjl/SzJ strain (NOD scid gamma, NSG) is a severely immunodeficient inbred laboratory mouse used for preclinical studies because it is amenable to engraftment with human cells. Combining scid and Il2rg^null mutations results in severe immunodeficiency by impairing the maturation, survival, and functionality of interleukin 2–dependent immune cells, including T, B, and natural killer lymphocytes. While NSG mice are reportedly resistant to developing spontaneous lymphomas/leukemias, there are reports of hematopoietic cancers developing. In this study, we characterized the immunophenotype of spontaneous lymphoma/leukemia in 12 NSG mice (20 to 38 weeks old). The mice had a combination of grossly enlarged thymus, spleen, or lymph nodes and variable histologic involvement of the bone marrow and other tissues. All 12 lymphomas were diffusely CD3, TDT, and CD4 positive, and 11 of 12 were also positive for CD8, which together was consistent with precursor T-cell lymphoblastic lymphoma/leukemia (pre-T-LBL). A subset of NSG tissues from all mice and neoplastic lymphocytes from 8 of 12 cases had strong immunoreactivity for retroviral p30 core protein, suggesting an association with a viral infection. These data highlight that NSG mice may develop T-cell lymphoma at low frequency, necessitating the recognition of this spontaneously arising disease when interpreting studies.