Assembly of (bio)polymers into long-range anisotropic nanostructured gels and aerogels is of great interest in advanced material engineering since it enables directional tuning of properties, such as diffusivity, light, heat, and sound propagation, cell proliferation, and mechanical properties. Here we present an approach toward anisotropic cellulose II gels and aerogels that employs specific diffusion and phase separation phenomena occurring during decelerated infusion of an antisolvent into isotropic supercooled solutions of cellulose in an ionic liquid to effectuate supramolecular assembly of cellulose in anisotropic colloidal network structures. At the example of the distillable ionic liquid 1,1,3,3-tetramethylguanidinium acetate, the antisolvent ethanol, and spherocylindrical porous molds, we demonstrate that the proposed facile, environmental-benign and versatile route affords gels and aerogels whose specific anisotropic nanomorphology and properties reflect the preferred supramolecular cellulose orientation during phase separation, which is perpendicular to the direction of antisolvent diffusion. Comprehensive X-ray scattering experiments revealed that the (aero)gels are composed of an interconnected, fibrous, highly crystalline (CrI ≈ 72%), cellulose II with a cross-sectional Guinier radius of the struts of about 2.5 nm, and an order parameter gradient from about 0.1 to 0.2. The obtained gels and aerogels feature high specific surface areas (350–630 m2 g–1) and excellent mechanical properties like high toughness (up to 471 kJ m–3 for a 60% compression, ρB = 80 mg cm–3) and resilience (up to 13.4 kJ m–3, ρB = 65 mg cm–3).