Thymic stromal lymphopoietin (TSLP) is a type I cytokine that plays essential roles in allergic/inflammatory skin and airway disorders, in helminth infections, and in regulating intestinal immunity. TSLP signals via IL-7Rα and a specific TSLPR subunit that is highly related to the common cytokine receptor γ chain, γ(c). Although TSLP has effects on a broad range of hematopoetic cells and can induce STAT5 phosphorylation, TSLP was reported to not signal via JAK kinases, and the mechanism by which TSLP regulates STAT5 phosphorylation has been unclear. We now demonstrate the role of JAK1 and JAK2 in TSLP-mediated STAT5 phosphorylation in mouse and human primary CD4(+) T cells, in contrast to the known activation of JAK1 and JAK3 by the related cytokine, IL-7. We also show that just as JAK1 interacts with IL-7Rα, JAK2 is associated with TSLPR protein. Moreover, we demonstrate the importance of STAT5 activation for TSLP-mediated survival and proliferation of CD4(+) T cells. These findings clarify the basis for TSLP-mediated signaling and provide an example wherein a cytokine uses JAK1 and JAK2 to mediate the activation of STAT5.