Antioxidant flavonoids have beer isolated from the flower of Rhododendron yedoense var. poukhanense. One new flavonoid and three known flavonoids, quercetin-5-O-${eta}$-glucopyranoside (1), quercetin (3), and quercitrin (4), were isolated from the butanol and ethyl acetate extracts of the plant. The new flavonoid was identified as myricitrin-5-methyl ether (2). The isolation of these flavonoids from this plant, for the first time, is a valuable finding. The flavonoids were evaluated for their antioxidant activities using 1,1-diphenyl-2-picrylhydrazyl free radical (DPPH), TBARS (thiobarbituric acid reactive substance) and superoxide anion radical (O$_2$) in the xanthine/xanthine oxidase assay system. In the DPPH scavenging assay, the IC$_{50}$ values were 4.5${pm}$0.48 ${mu}$M for compound 2 and 9.7${pm}$0.29 ${mu}$M for compound 3, which showed an antioxidant activity approximately 1.5${sim}$2 times higher than the antioxidant activity of ${alpha}$-tocopherol (9.8${pm}$0.94 ${mu}$M). Additionally, the antioxidant activities of myricitrin-5-methyl ether (2) (IC$_{50}$ = 1.7${pm}$0.22 ${mu}$M) and quercetrin (4) (IC$_{50}$ = 1.9${pm}$0.63 ${mu}$M) were higher than that of L-ascorbic acid (IC$_{50}$ = 7.4${pm}$0.63 ${mu}$M) when evaluated using a TBARS assay. Compound 2 showed high activity in both the inhibition of xanthine oxidase (1.1${pm}$0.21 ${mu}$M) and in the activation of superoxide scavenging.