Although microRNAs (miRs) have been well recognized to play an important role in the pathogenesis of organ fibrosis, there is a lack of evidence as to whether miRs directly regulate the differentiation of myofibroblasts, the putative effector cells during pathological fibrogenesis. In this study, we found that levels of miR-27a-3p were up-regulated in transforming growth factor-β1-treated human lung fibroblasts in a Smad2/3-dependent manner and in fibroblasts isolated from lungs of mice with experimental pulmonary fibrosis. However, both basal and transforming growth factor-β1-induced expression of miR-27a-3p were reduced in lung fibroblasts from patients with idiopathic pulmonary fibrosis compared with that from normal control subjects. Overexpression of miR-27a-3p inhibited, whereas knockdown of miR-27a-3p enhanced, the differentiation of lung fibroblasts into myofibroblasts. We found that miR-27a-3p directly targeted the phenotypic marker of myofibroblasts, a-smooth muscle actin, and two key Smad transcription factors, Smad2 and Smad4. More importantly, we found that therapeutic expression of miR-27a-3p in mouse lungs through lentiviral delivery diminished bleomycin-induced lung fibrosis. In conclusion, our data suggest that miR-27a-3p functions via a negative-feedback mechanism in inhibiting lung fibrosis. This study also indicates that targeting miR-27a-3p is a novel therapeutic approach to treat fibrotic organ disorders, including lung fibrosis.