Fibroblast Growth Factor 9 (FGF9) is necessary for fetal lung development and is expressed by epithelium and mesothelium. We evaluated the role of FGF9 overexpression on adenoviral-induced pleural injury in vivo and determined the biological effects of FGF9 on mesothelial cells in vitro. We assessed the expression of FGF9 and FGF receptors by mesothelial cells in both Human and mouse lungs. Intrapleural injection of an adenovirus expressing human FGF9 (AdFGF9) or a control adenovirus (Adcont) was performed. Mice were sacrificed at day 3, day 5 and day 14. Expression of FGF9 and markers of inflammation and myofibroblastic differentiation was studied by qPCR and immunohistochemistry. In vitro, rat mesothelial cells were stimulated with FGF9 (20ng/ml) and we assessed it effect on proliferation, survival, migration and differentiation. FGF9 was expressed by mesothelial cells in human IPF. FGF receptors, mainly FGFR3, were expressed by mesothelial cells in vivo in human and mice. Adcont instillation induced diffuse pleural thickening appearing at day 5, maximal at day 14. The altered pleura cells strongly expressed alpha-smooth muscle actin and collagen. AdFGF9 injection induced maximal FGF9 expression at day 5 which lasted until day 14. FGF9 overexpression prevented pleural thickening, collagen and fibronectin accumulation, and myofibroblastic differentiation of mesothelial cells. In vitro, FGF9 decreased mesothelial cell migration and inhibited the differentiating effect of TGF-ß1. We conclude that FGF9 has a potential antifibrotic effect on mesothelial cells.