Antioxidant fractions from Ophioglossum thermale were extracted with five different polar solvents using a Soxhlet type extractor. The total phenolic content of the extracts was determined by the Folin-Ciocalteu method. The ethyl acetate fraction of O. thermale was found to contain maximum phenolics. The dried fractions were screened for their antioxidant activity potential using in vitro model systems such as 1,1-diphenyl-2-picryl hydrazyl (DPPH), nitroblue tetrazolium (NBT) and lipid-peroxidation reduction at different concentrations. Results revealed that the EtOAc fraction exhibited the best performance in the DPPH assay, NBT assay and lipid peroxidation. All fractions showed more potent antioxidant capacity than green tea extract, a well-known antioxidant. Furthermore, the EtOAc fraction has the highest total phenolic content (475.65 mg of EGCG/g). In addition, the EtOAc fraction at 0.005% and 0.01% (g/100 ml) also significantly inhibited UVB irradiation-induced ROS generation in human dermal fibroblasts (HDFs). In a carrageenan-induced edema model, the EtOAc fraction showed an inhibitory effect (21.5%, p < 0.05) at 200 mg/kg (p.o.) after 300 min administration. Consequently, 3-O-methylquercetin (3MQ) was also isolated from the antioxidative EtOAc fraction. The data obtained using the above in vitro and in vivo tests suggest that the antioxidant activity of O. thermale and its anti-inflammatory effect on carrageenan-induced acute inflammation can be attributed to its ameliorating effect on oxidative damage, and thus it has great potential as a source for natural health products. To the best of our knowledge, this is the first report on the antioxidant activity of different polar extracts from O. thermale.