In many physiological and disease processes, TGF-β usurps branches of MAP kinase pathways in conjunction with Smads to induce apoptosis and epithelial-to-mesenchymal transition, but the detailed mechanism of how a MAP kinase cascade is activated by TGF-β receptors is not clear. We report here that TRAF6 is specifically required for the Smad-independent activation of JNK and p38, and its carboxyl TRAF homology domain physically interacts with TGF-β receptors. TGF-β induces K63-linked ubiquitination of TRAF6 and promotes association between TRAF6 and TAK1. Our results indicate that TGF-β activates JNK and p38 through a mechanism similar to that operating in the interleukin-1β/Toll-like receptor pathway.