In living organisms, nanoscale building blocks are well organized into a nearly defect-free arrangement, leading to sophisticated hierarchical structures that exhibit high mechanical strengths and functionalities. Cellulose nanofibrils are representative building blocks in nature and have raised great interest as excellent structural materials, originating from the most abundant bioresource, wood biomass. Here, we report integration controls of the cellulose nanofibrils that have been completely dispersed in water through the specific surface-carboxylation using 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO) as a catalyst. The dispersed nanofibrils spontaneously align in water. The integration controls of the self-aligned nanofibrils, i.e., careful adjustment of the pH and evaporation of the solvent in the nanofibril dispersions, produce a wide range of artificial bulk materials with outstanding properties. Examples include unprecedentedly stiff hydrogels that are freestanding with a water content of 99.9%, ultralow-density, tough aerogels with large surface areas, and transparent films with exceptionally high oxygen-barrier properties. These materials are expected to further develop as robust frameworks of polymer nanocomposites or high-capacity supports of catalysts and the other functional materials. For the sustainable development of society, we propose a methodology for producing sophisticated, high-performance bio-based materials.