Smad2, Smad3 and Smad4 proteins are considered to be key mediators of transforming growth factor-β (TGF-β) signaling. However, the identities of the Smad partners mediating TGF-β signaling are not fully understood. Here, we show that RNA-binding protein with multiple splicing (RBPMS), a member of the RNA-binding protein family, physically interacts with Smad2, Smad3 and Smad4 both in vitro and in vivo. The presence of TGF-β increases the binding of RBPMS with these Smad proteins. Consistent with the binding results, overexpression of RBPMS enhances Smad-dependent transcriptional activity in a TGF-β-dependent manner, whereas knockdown of RBPMS decreases this activity. RBPMS interacts with TGF-β receptor type I (TβR-I), increases phosphorylation of C-terminal SSXS regions in Smad2 and Smad3, and promotes the nuclear accumulation of the Smad proteins. Moreover, RBPMS fails to enhance the transcriptional activity of Smad2 and Smad3 that lack the C-terminal phosphorylation sites. Our data provide the first evidence for an RNA-binding protein playing a role in regulation of Smad-mediated transcriptional activity and suggest that RBPMS stimulates Smad-mediated transactivation possibly through enhanced phosphorylation of Smad2 and Smad3 at the C-terminus and promotion of the nuclear accumulation of the Smad proteins.