Significance The recalcitrant nature of the plant cell wall presents a significant challenge in the industrial processing of biomass. Poor understanding of plant polysaccharide biosynthesis impedes efforts to engineer cell walls susceptible to efficient and unnatural pathways of degradation. Despite numerous genetic and in vitro studies of the xyloglucan xylosyltransferases (XXT1, XXT2, and XXT5), the specific roles of each in the xylosylation of the xyloglucan backbone is unclear. On the basis of steric constraints imposed by the active-site cleft of structures presented here, we propose a multienzyme complex capable of producing the xylosylation patterns of native xyloglucans. This model significantly extends our limited understanding of branched polysaccharide biosynthesis.