Water transport from roots to leaves through xylem is important for plant growth and development. Defects in water transport can cause drought stress, even when there is adequate water in the soil. Here, we identi-fied the maize (Zea mays) wilty5 (wi5) mutant, which ex-hibits marked dwarfing and leaf wilting throughout most of its life cycle under normal growth conditions. wilty5 seed-lings exhibited lower xylem conductivity and wilted more rapidly under drought, NaCl, and high temperature treat-ments than wild-type plants. Map-based cloning revealed that WI5 encodes an active endo-1,4-β-xylanase from gly-cosyl dehydration family 10, which mainly functions in degrading and reorganizing cell wall xylan. Reverse-transcription polymerase chain reaction andβ-glucuronidase assays revealed that WI5 is highly expressed in stems, especially in intodes undergoing secondary wall as-sembly. RNA sequencing suggested that WI5 plays a unique role in intode growth. Immunohistochemistry and electron microscopy confirmed that wi5 is defective in xylan deposition and secondary cell wall thickening. Lignin deposition and xylan content were markedly re-duced in wi5 compared to the wild-type plants. Our results suggest that WI5 functions in xylem cell wall thickening through its xylanase activity and thereby regulates xylem water transport, the drought stress response, and plant growth in maize.