Hypericum androsaemum is a medicinal plant species containing many polyphenolic compounds, namely flavonoids and phenolic acids. Since polyphenolic compounds have high antioxidant potential, the ability of H. androsaemum infusion to act as a scavenger of reactive oxygen species (superoxide radical, hydroxyl radical and hypochlorous acid) was investigated. Superoxide radical was generated by the xanthine/xanthine oxidase and phenazine methosulphate/NADH systems. The infusion-mediated prevention of nitroblue tetrazolium reduction by the superoxide radical was used as the measured endpoint. Hydroxyl radical was generated by the Fe³⁺-EDTA/ascorbate Fenton system, and assayed by evaluating deoxyribose degradation using the thiobarbituric acid method. Hypochlorous acid scavenging activity was tested by measuring the inhibition of hypochlorous acid-induced 5-thio-2-nitrobenzoic acid oxidation to 5,5′-dithiobis(2-nitrobenzoic acid). The tested infusion mainly exhibited a potent scavenging effect on superoxide radicals (although a noncompetitive inhibitory effect on xanthine oxidase was also observed). The infusion also acted as a moderate scavenger of hydroxyl radicals and hypochlorous acid. A phytochemical study of the infusion was also undertaken, and nine phenolic compounds were identified.