Atrial fibrosis plays a critical role in atrial fibrillation (AF) by the transforming growth factor (TGF)-β1/Smad pathway. The disordered differentiation, proliferation, migration and collagen deposition of atrial fibroblasts play significant roles in atrial fibrosis. Mitsugumin (MG)53 is predominantly expressed in myocardium of rodents and has multiple biological functions. However, the role of MG53 in cardiac fibrosis remains unclear. This study provided clinical and experimental evidence for the involvement of MG53 in atrial fibrosis in humans and atrial fibrosis phenotype in cultured rat atrial fibroblasts. In atrial tissue from patients we demonstrated that MG53 was expressed in human atrium. Expression of MG53 increased with the extent of atrial fibrosis, which could induce AF. In cultured atrial fibroblasts, depletion of MG53 by siRNA caused down-regulation of the TGF-β1/Smad pathway, while overexpression of MG53 by adenovirus up-regulated the pathway. MG53 regulated the proliferation and migration of atrial fibroblasts. Besides, exogenous TGF-β1 suppressed expression of MG53. In conclusion, we demonstrated that MG53 was expressed in human atrium, and may be a potential upstream of the TGF-β1/Smad pathway in human atrium and rat atrial fibroblasts. This suggests that MG53 is a potential regulator of atrial fibrosis induced by the TGF-β1/Smad pathway in patients with AF.