The present study demonstrated that murine protein serine/threonine kinase 38 (MPK38) coimmunoprecipitates with Smad proteins (Smad2, -3, -4, and -7) and that this association is mediated by the catalytic kinase domain of MPK38. The association between MPK38 and Smad2, -3, and -4 was significantly increased by TGF-β or ASK1 signals, whereas these signals decreased association of MPK38 with Smad7. MPK38 stimulated TGF-β-induced transcription required for TGF-β-mediated biological functions, such as apoptosis and cell growth arrest, in a kinase-dependent manner. Knockdown of endogenous MPK38 showed an opposite effect, inhibiting TGF-β signaling. MPK38-mediated phosphorylation of Smad proteins (Ser(245) of Smad2, Ser(204) of Smad3, Ser(343) of Smad4, and Thr(96) of Smad7) was also found to be crucial to the positive regulation of TGF-β signaling induced by MPK38. In addition, MPK38 enhanced nuclear translocation of Smad3, as well as redistribution of Smad7 from the nucleus to the cytoplasm, in response to TGF-β. Together, these results indicate that MPK38 functions as a stimulator of TGF-β signaling through direct interaction with and phosphorylation of Smad proteins.