Signaling by type I cytokines involves the formation of receptor homodimers, heterodimers or higher order receptor oligomers. Here we report the cloning of a type I cytokine receptor subunit that is most closely related to the common cytokine receptor γ chain (γ c). Binding and crosslinking experiments demonstrate that this protein is the receptor for a recently described interleukin 7 (IL-7)-like factor, thymic stromal lymphopoietin (TSLP). Binding of TSLP to the thymic stromal lymphopoietin receptor (TSLPR) is increased markedly in the presence of the IL-7 receptor α chain (IL-7Rα). IL-7Rα–expressing but not parental 32D cells proliferate in the presence of exogenous TSLP. Moreover, a combination of IL-7Rα and TSLPR is required for TSLP-dependent activation of a STAT5-dependent reporter construct. Thus it is shown that IL-7Rα is a component of both the IL-7 and TSLP receptors, which helps to explain why deletion of the gene that encodes IL-7Rα affects the lymphoid system more severely than deletion of the gene encoding IL-7 does. Cloning of TSLPR should facilitate an understanding of TSLP function and its signaling mechanism.