Nasal polyps are chronic inflammatory conditions characterized by myofibroblast differentiation and extracelluar matrix accumulation. The major catechin from green tea is (−)-epigallocatechin-3-gallate (EGCG), which has garnered attention for its potential to prevent oxidative stress-related diseases. The purpose of this study was twofold: (i) to determine the effect of EGCG on fibroblast differentiation into myofibroblasts and extracellular matrix accumulation in transforming growth factor (TGF)-β1-induced nasal polyp-derived fibroblasts (NPDFs) and (ii) to determine if the antioxidative effect of EGCG on reactive oxygen species (ROS) production in TGF-β1-induced NPDFs is involved in the aforementioned processes. TGF-β1-induced NPDFs were treated with or without EGCG. α-smooth muscle actin (α-SMA) and collagen type I mRNA were analyzed by reverse transcription-polymerase chain reaction. α-SMA protein was also detected using immunofluorescent staining. The amount of total soluble collagen was analyzed by Sircol collagen assay. ROS activity was measured by the nitroblue tetrazolium reduction assay and visualized by fluorescent microscopy. EGCG significantly inhibited expressions of α-SMA and collagen type I mRNA and reduced α-SMA and collagen protein levels at concentrations of 10–20 µg/mL. EGCG also inhibited TGF-β1-induced ROS production at the same concentrations. These results suggest the possibility that EGCG may be effective at inhibiting the development of nasal polyps through an anti-oxidant effect. Copyright © 2013 John Wiley & Sons, Ltd.