Although UDP-glucuronic acid decarboxylases (UXSs) have been well studied to catalyze the conversion of UDP-glucuronic acid into UDP-xylose, their biological roles in grasses remains largely unknown. The rice (Oryza sativa) genome contains six UXSs, but none of them has been genetically characterized. Here, we reported on the characterization of a novel rice fragile culm mutant, fc18, which exhibited brittleness with altered cell wall and pleiotropic defects in growth. Map-based cloning and transgenic analyses revealed that the FC18 encodes a cytosol-localized OsUXS3 and is widely expressed with higher levels in xylan-rich tissues. Monosaccharide analysis showed that the xylose level decreased in fc18, and cell wall fractions determinations confirmed that the xylan content in fc18 declined, suggesting that UDP-xylose from FC18 participates in xylan biosynthesis. Moreover, the fc18 mutant displayed defective cellulose properties, which led to an enhancement in biomass saccharification. Furthermore, genes involved in sugar metabolism and phytohormone signal transduction were largely altered in fc18. Consistently, the fc18 mutant exhibited significantly reduced free auxin (indole-3-acetic acid, IAA) content and lower expression levels of PIN family genes compared to wild-type. Our work underpins the physiological roles of FC18/UXS3 in xylan biosynthesis, cellulose deposition, and plant growth in rice.