Transforming growth factor-β (TGF-β) is a multifunctional cytokine that regulates embryonic development and tissue homeostasis; however, aberrations of its activity occur in cancer^,. TGF-β signals through its Type II and Type I receptors (TβRII and TβRI) causing phosphorylation of Smad proteins^,. TGF-β-associated kinase 1 (TAK1), a member of the mitogen-activated protein kinase kinase kinase (MAPKKK) family, was originally identified as an effector of TGF-β-induced p38 activation. However, the molecular mechanisms for its activation are unknown. Here we report that the ubiquitin ligase (E3) TRAF6 interacts with a consensus motif present in TβRI. The TβRI–TRAF6 interaction is required for TGF-β-induced autoubiquitylation of TRAF6 and subsequent activation of the TAK1–p38/JNK pathway, which leads to apoptosis. TβRI kinase activity is required for activation of the canonical Smad pathway, whereas E3 activity of TRAF6 regulates the activation of TAK1 in a receptor kinase-independent manner. Intriguingly, TGF-β-induced TRAF6-mediated Lys 63-linked polyubiquitylation of TAK1 Lys 34 correlates with TAK1 activation. Our data show that TGF-β specifically activates TAK1 through interaction of TβRI with TRAF6, whereas activation of Smad2 is not dependent on TRAF6.