Abstract text: Cellulose is a linear glucose polymer that is assembled into fibrils to form the load bearing component of plant cell walls. While cellulose fibers have been characterized at high resolution in a dehydrated form, little is known about the fiber architecture in a native-like, hydrated state. We employed cryo-electron microscopy, fluorescence imaging, and molecular dynamics simulations to investigate the structure of a never-dried cellulose fibril. Our results reveal a supramolecular assembly of glucan chains stabilized by structural water molecules. The hydrated fiber is flexible, enabling random bending and rotation around the fiber axis and twisting of individual glucan chains. Solid-state NMR confirms the presence of internal water molecules in the cellulose fiber and their slow exchange with bulk solvent. Molecular dynamics simulations support the accumulation of regularly spaced water molecules along the fiber axis and provide insights into fibrillogenesis in an aqueous milieu. Our results provide an atomistic model of a native-like cellulose fiber that will inform current models of cell wall assembly.