The collapse of collagenous networks with aging results in comprehensive changes in the functional properties of skin. α-Lipoic acid (LA) is known to possess beneficial effects against skin aging, effects often presumed to be its antioxidant potential. However, the effects of LA on fibrillogenesis in dermal fibroblasts have not been adequately assessed. In this study, we demonstrated for the first time that LA enhances the biosynthesis of new collagen in normal human dermal fibroblasts (NHDFs). By using a quantitative dye-binding method and immunochemical approaches, we showed that LA effectively increased the expression and subsequently the deposition of type I collagen in NHDFs. LA also facilitated the expression of a collagen-processing enzyme, prolyl-4-hydroxylase, pointing to the existence of a posttranslational mechanism among the LA-mediated effects on collagen synthesis. In addition, we determined that both Smad 2/3 were rapidly phosphorylated by treatment with LA within 30 min, indicating that LA enhances type I collagen synthesis through the activation of Smad signaling. Pretreatment of SB431542, a specific transforming growth factor-β (TGF-β) receptor type I (TβRI) kinase inhibitor, blocked LA-mediated Smad 2/3 phosphorylations and both type I collagen and prolyl-4-hydroxylase expression, suggesting that LA-mediated cell responses are regulated by TβRI kinase-dependent pathway. Levels of TGF-β secretion after 4 hr of treatment with LA were not remarkably elevated, indicating that LA may be able to mimic TGF-β-mediated cell response. The study results produced new insights into the molecular pharmacology of LA in NHDFs, with potential applications in the treatment of aging skin.