JAK3-activating mutations are commonly seen in chronic or acute hematologic malignancies affecting the myeloid, megakaryocytic, lymphoid, and natural killer (NK) cell compartment. Overexpression models of mutant JAK3 or pharmacologic inhibition of its kinase activity have highlighted the role that these constitutively activated mutants play in the T-cell, NK cell, and megakaryocytic lineages, but to date, the functional impact of JAK3 mutations at an endogenous level remains unknown. Here, we report a JAK3(A572V) knockin mouse model and demonstrate that activated JAK3 leads to a progressive and dose-dependent expansion of CD8(+) T cells in the periphery before colonization of the bone marrow. This phenotype is dependent on the gamma c chain of cytokine receptors and presents several features of the human leukemic form of cutaneous T-cell lymphoma (L-CTCL), including skin involvements. We also showed that the JAK3(A572V)-positive malignant cells are transplantable and phenotypically heterogeneous in bone marrow transplantation assays. Interestingly, we revealed that activated JAK3 functionally cooperates with partial trisomy 21 in vivo to enhance the L-CTCL phenotype, ultimately leading to a lethal and fully penetrant disorder. Finally, we assessed the efficacy of JAK3 inhibition and showed that CTCL JAK3(A572V)-positive T cells are sensitive to tofacitinib, which provides additional preclinical insights into the use of JAK3 inhibitors in these disorders. Altogether, this JAK3(A572V) knockin model is a relevant new tool for testing the efficacy of JAK inhibitors in JAK3-related hematopoietic malignancies.