Background. The p38 mitogen-activated protein kinase (p38 MAPK) is an important intracellular signal transduction pathway involved in TGF-beta 1-induced epithelial-mesenchymal transition (EMT). Sema4C, a member of the semaphorin family, was found to be essential for the activation of p38 MAPK. However, the role of Sema4C in promoting TGF-beta 1-induced EMT is unclear. Methods. Renal fibrosis was induced by 5/6 subtotal nephrectomy rat model. In vitro, Sema4C was induced in human proximal tubular epithelial cells (HKC) by treatment with TGF-beta 1, or was inhibited by siRNA or was over-expressed by Sema4C transfection. The selective p38 MAPK inhibitor, SB203580, was administered to inhibit the p38 pathway. The expression of Sema4C, the markers of EMT, p38 phosphorylation and fibronectin secretion were measured by western blotting, immunohistochemistry, immunocytochemistry or enzyme-linked immunosorbent assay. Results. The expression of Sema4C increased in HKC cells that were treated with TGF-beta 1. Knockdown of Sema4C potently inhibited phosphorylation of p38 MAPK and reversed TGF-beta 1-induced EMT. Over-expression of Sema4C via Sema4C transfection elicited p38 MAPK phosphorylation and promoted EMT. The effects of Sema4C during EMT were blocked by a p38-specific inhibitor. In vivo, the expression of Sema4C increased in the tubular epithelia of 5/6-nephrectomized rats and human fibrotic renal tissue, and similar localization of phosphorylated p38 and Sema4C was demonstrated by immunohistochemistry on serial sections. Conclusions. Our findings suggest that Sema4C plays an important role in TGF-beta 1-induced EMT through activation of p38 MAPK in proximal tubular epithelial cells.