Psidium cattleianum J. Sabine (Myrtaceae) is a traditional medicinal plant in French Polynesia. The leaves and roots possess many medicinal properties. These effects may be correlated with the presence of antioxidant compounds. Seven flavonoids along with a benzoic acid were isolated from the leaves of P. cattleianum. The compounds indicated strong antioxidant and radical-scavenging activities in ALP, DPPH(·), ABTS(·-) and ORAC assays. This study demonstrates that the leaves of P. cattleianum possess main compounds with interesting antioxidant and radical-scavenging activities, as clarified by four biological assays. Our findings may justify the use of these leaves in the traditional medicine of French Polynesia. Among the total eight known compounds, reynoutrin and luteolin were isolated for the first time from the genus Psidium.