The increasing prevalence of hyperuricemia is most likely caused by the “Westernized” lifestyle and environment. Excessive intake of purines increases serum uric acid (SUA) concentration and is thought to be one factor causative of hyperuricemia. This study was performed to examine the effect of agarose on the elevation of SUA concentration (experimental hyperuricemia) induced by dietary ribonucleic acid (RNA). In vitro, agarose significantly decreased the digestion rate of RNA by ribonuclease A (RNase A) and uptake of ¹⁴C-labeled adenosine and adenosine-5’-monophosphate (5’-AMP) in the rat jejunum, compared with the fiber-free control. In addition, in vivo experiments with 5-week-old male Wistar rats fed diets with a 3% (w/w) yeast RNA and without dietary fiber (DF) or with 5% (w/w) DF (cellulose or agarose) for five days revealed that the cellulose and agarose groups significantly lowered SUA and allantoin concentrations and urinary excretions of related compounds, and increased the amount of RNA excreted into feces, compared with the fiber-free control group, except for the amount of urinary allantoin excretion in the cellulose group. The agarose group had significantly decreased SUA and allantoin concentrations, and increased uric acid clearance and the amount of RNA excreted into the feces, compared with the cellulose group (positive control). The present results suggest that experimental hyperuricemia induced by dietary RNA in rats can be suppressed by agarose and cellulose, and that the suppressive effect of agarose is greater than that of cellulose.