The purpose of this study was to evaluate the inhibitory effect of renierol, extracted from marine sponge Halicdona.SP., on xanthine oxidase (XO) and its hypouricemic effect in vivo. Renierol and a positive control, allopurinol, were tested for their effects on XO activity by measuring the formation of uric acid and superoxide radical from xanthine. Renierol inhibited XO in a concentration-dependent and competitive manner. IC₅₀ value was 1.85 μg·ml^- 1 through the measuring of uric acid and was 1.36 μg.ml^- 1 through the measuring of superoxide radical. Renierol was found to have an in vivo hypouricemic activity against potassium oxonate-induced hyperuricaemia in mice. After oral administration of renierol at doses of 10, 20 and 30 mg.kg^- 1, there was a significant decrease in the serum urate level (4.08 ± 0.09 mg.dl^- 1, P < 0.01), (3.47 ± 0.11 mg.dl^- 1, P < 0.01) and (3.12 ± 0.08 mg.dl^- 1, P < 0.01), when compared to the hyperuricaemic control (6.74 ± 0.23 mg.dl^- 1). Renierol was a potent XO inhibitor with hypouricemic activity in mice.