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Biomimetic hydrogels offer a new platform for hierarchical structure-controlled, tough, biocompatible, mechanically tunable, and printable gels for regenerative medicine. Herein, we report for the first time the detailed effects of various kinds of nanocellulose, namely, bacterial nanocellulose, cellulose nanofibers, and cellulose nanocrystals on the morphology, structure-property relationship, and 3D printability of the photochemically cross-linked regenerated silk fibroin (RSF)/nanocellulose composite hydrogels. The hierarchical structure of fabricated biomimetic hydrogels was both qualitatively and quantitatively investigated by scanning electron microscopy and small/ultrasmall-angle neutron scattering, whereas their mechanical properties were assessed using rheology, tensile, and indentation tests. The micropore size and interhydrophobic domain distance of fabricated hydrogels were tuned in the range of 1.8-9.2 μm and 4.5-17.7 nm, respectively. The composite hydrogels exhibit superior viscoelastic, compressive, and tensile mechanical properties compared to pristine RSF hydrogel, where the shear storage modulus, compression modulus, young's modulus, and tensile toughness were tuned in the range of 0.4-1.4, 1.3-3.6, 2.2-14.0 MPa, and 16.7-108.3 kJ/m3, respectively. Moreover, the obtained mechanical modulus of the composite hydrogels in terms of shear, tensile, and compression are comparable to articular cartilage (0.4-1.6 MPa), native femoral artery (∼9.0 MPa), and human medial meniscus (∼1.0 MPa) tissues, respectively, which demonstrate their potential for a wide range of tissue engineering applications. The whisker form of nanocellulose was observed to enhance the printability of composite hydrogels, whereas the fiber form enhanced the overall toughness of the composite hydrogels and promoted the fibroblast cell attachment, viability, and proliferation. The results presented here have implications for both fundamental understanding and potential applications of