Cell division autoantigen 1 (CDA1) modulates cell proliferation and transforming growth factor-β (TGF-β) signaling in a number of cellular systems; here we found that its levels were elevated in the kidneys of two animal models of diabetic renal disease. The localization of CDA1 to tubular cells and podocytes in human kidney sections was similar to that seen in the rodent models. CDA1 small interfering RNA knockdown markedly attenuated, whereas its overexpression increased TGF-β signaling, modulating the expression of TGF-β, TGF-β receptors, connective tissue growth factor, collagen types I, III, IV, and fibronectin genes in HK-2 cells. CDA1 and TGF-β together were synergistic in stimulating TGF-β signaling and target gene expression. CDA1 knockdown effectively blocked TGF-β-stimulated expression of collagen genes. This was due to its ability to modulate the TGF-β type I, but not the type II, receptor, leading to increased phosphorylation of Smad3 and extracellular signal-regulated kinase mitogen-activated protein kinase. Furthermore, the Smad3 inhibitor, SIS3, markedly attenuated the activities of CDA1 in stimulating TGF-β signaling as well as gene expression of collagens I, III, and IV. Thus, our in vitro and in vivo findings show that CDA1 has a critical role in TGF-β signaling in the kidney.