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Icing in the Cake: Water in Nanoscopic Confinement by Cellulose Water has profound effects in both cellulosic materials and the plant cell wall. In particular, water was recently shown to reside at the fibril–fibril interfaces inside cellulose fibril aggregates where it attains a structural role. We use molecular dynamics simulations to investigate the properties of water confined by cellulose surfaces at a specific and conceptually well-defined distance L. We study different crystalline faces of cellulose interacting with the water molecules and vary the confinement so that the water region changes from submonomolecular to essentially bulk. We find that confinement hinders molecular motions. In particular, the translational self-diffusion coefficient D exhibits a dramatic divergence and slows by up to three orders of magnitude from its bulk value for a defective monolayer of water. In the same regime, water also attains a strong preferential orientation with regards to the confining surfaces. The mass density of the water layer evolves with L in a nonmonotonic and intriguing manner. As pore size decreases, at roughly monolayer separation, the density first increases from its bulk value so that it approaches the densities of high-pressure forms of ice. When water becomes sparser than monomolecular, its mass density sharply drops as it should for a defective layer. In this defective layer, the reorientation is not only slow, but completely anisotropic. These observations on the atomistic scale highlight the unique ways cellulose and water, two very abundant materials interact with each other. https://pubs.acs.org/doi/10.1021/acs.jpcb.5c06900