Suppressor of cytokine signaling (SOCS)3 is a major negative feedback regulator of signal transducer and activator of transcription (STAT)3-activating cytokines. Transgenic mouse studies indicate that high levels of SOCS3 in T cells result in type 2 T helper cell (Th2) skewing and lead to hypersensitivity to allergic diseases. To define the physiological roles of SOCS3 in T cells, we generated T cell-specific SOCS3 conditional knockout mice. We found that the mice lacking SOCS3 in T cells showed reduced immune responses not only to ovalbumin-induced airway hyperresponsiveness but also to Leishmania major infection. In vitro, SOCS3-deficient CD4+ T cells produced more transforming growth factor (TGF)-beta1 and interleukin (IL)-10, but less IL-4 than control T cells, suggesting preferential Th3-like differentiation. We found that STAT3 positively regulates TGF-beta1 promoter activity depending on the potential STAT3 binding sites. Furthermore, chromatin immunoprecipitation assay revealed that more STAT3 was recruited to the TGF-beta1 promoter in SOCS3-deficient T cells than in control T cells. The activated STAT3 enhanced TGF-beta1 and IL-10 expression in T cells, whereas the dominant-negative form of STAT3 suppressed these. From these findings, we propose that SOCS3 regulates the production of the immunoregulatory cytokines TGF-beta1 and IL-10 through modulating STAT3 activation.