In higher plants, cellulose is synthesized by plasma membrane-localized cellulose synthase complexes (CSCs). Arabidopsis thaliana GH9A1/KORRIGAN1 is a membrane-bound, family 9 glycosyl hydrolase that is important for cellulose synthesis in both primary and secondary cell walls. Most previously identified korrigan1 mutants show severe phenotypes such as embryo lethality; therefore, the role of GH9A1 in cellulose synthesis remains unclear. Here, we report a novel A577V missense mutation, designated jiaoyao1 (jia1), in the second of the glycosyl hydrolase family 9 active site signature motifs in GH9A1. jia1 is defective in cell expansion in dark-grown hypocotyls, roots, and adult plants. Consistent with its defect in cell expansion, this mutation in GH9A1 resulted in reduced cellulose content and reduced CSC velocity at the plasma membrane. Green fluorescent protein-GH9A1 is associated with CSCs at multiple locations, including the plasma membrane, Golgi, trans-Golgi network, and small CESA-containing compartments or microtubule-associated cellulose synthase compartments, indicating a tight association between GH9A1 and CSCs. GH9A1A577V abolishes the endoglucanase activity of GH9A1 in vitro but does not affect its interaction with CESAs in vitro, suggesting that endoglucanase activity is important for cellulose synthesis. Interestingly, jia1 results in both cellulose microfibril and microtubule disorganization. Our study establishes the important role of endoglucanase in cellulose synthesis and cellulose microfibril organization in plants.