Rationale:: Tachycardia-induced atrial fibrosis is a hallmark of structural remodeling of atrial fibrillation (AF). The molecular mechanisms underlying the AF-induced atrial fibrosis remain unclear. Objective:: To determine the role of angiotensin II (Ang II)/Ang II type 1 (AT ₁ ) receptor–coupled transforming growth factor (TGF)-β ₁ /Smad signaling pathway in the AF-induced atrial fibrosis. Methods and Results:: Rapid atrial pacing (1000 ppm) was applied to the left atrium of rabbit heart to induce atrial fibrillation and fibrosis. Quantitative PCR and Western blot analysis revealed that rapid atrial pacing caused a marked increase in the expression of Ang II, TGF-β ₁ , phosphorylated Smad2/3 (P-Smad2/3), Arkadia, and hydroxyproline synthesis. However, the expression of Smad7, a key endogenous antagonist of the TGF-β ₁ /Smad-mediated fibrosis, was significantly decreased. These changes were dose-dependently reversed by AT ₁ receptor antagonist losartan, implicating the involvement of AF-induced release of Ang II and activation of AT ₁ receptor–specific pathway. In the adult rabbit cardiac fibroblasts, Ang II increased the expression of TGF-β ₁ , P-Smad2/3, Smad4, Arkadia, and collagen I synthesis and significantly reduced Smad7 expression. These effects of Ang II were reversed by losartan but not by the AT ₂ antagonist (PD123319). In addition, extracellular signal-regulated kinase inhibitor and anti–TGF-β ₁ antibody also blocked the Ang II–induced downregulation of Smad7. Silencing of Smad7 gene by small interfering RNA abolished the antagonism of losartan on the fibrogenic effects of Ang II on cardiac fibroblasts, whereas overexpression of Smad7 blocked Ang II–induced increase in collagen I synthesis. Conclusions:: Ang II/AT ₁ receptor–specific activation of Arkadia-mediated poly-ubiquitination and degradation of Smad7 may decrease the inhibitory feedback regulation of TGF-β ₁ /Smad signaling and serves as a key mechanism for AF-induced atrial fibrosis.